[ImgBot] optimizes images (#1) (travis-ci#1820)

*Total -- 2,600.21kb -> 1,388.19kb (46.61%)

/images/enable.png -- 236.80kb -> 2.62kb (98.89%)
/images/apps/weblint.jpg -- 22.29kb -> 5.30kb (76.22%)
/images/apps/travisify-node.jpg -- 24.80kb -> 5.94kb (76.04%)
/images/apps/jarvis.jpg -- 25.69kb -> 6.68kb (73.99%)
/images/apps/chrome-github-status.jpg -- 27.51kb -> 7.24kb (73.7%)
/images/apps/siren-windows-phone.jpg -- 31.89kb -> 8.44kb (73.53%)
/images/apps/cctray.jpg -- 20.66kb -> 6.70kb (67.59%)
/images/apps/travis-encrypt.jpg -- 20.61kb -> 6.70kb (67.5%)
/images/apps/firefox-travis-lite.jpg -- 14.01kb -> 4.56kb (67.49%)
/images/apps/travis-tools.jpg -- 16.60kb -> 5.41kb (67.4%)
/images/apps/travisify-ruby.jpg -- 20.29kb -> 6.63kb (67.32%)
/images/apps/travis-surveillance.jpg -- 20.04kb -> 6.57kb (67.19%)
/images/apps/travis-inside.jpg -- 16.03kb -> 5.26kb (67.18%)
/images/apps/cron.jpg -- 20.86kb -> 6.85kb (67.16%)
/images/apps/chrome-my-travis.jpg -- 18.07kb -> 5.94kb (67.15%)
/images/apps/travis-build-watcher.jpg -- 19.30kb -> 6.35kb (67.09%)
/images/apps/ducksboard.jpg -- 19.54kb -> 6.47kb (66.89%)
/images/apps/chrome-github-travis.jpg -- 19.96kb -> 6.62kb (66.84%)
/images/apps/comradetravis.jpg -- 23.63kb -> 7.84kb (66.8%)
/images/apps/travis-light.jpg -- 20.01kb -> 6.66kb (66.73%)
/images/apps/siren-windows.jpg -- 22.13kb -> 7.37kb (66.72%)
/images/apps/status-gravatar.jpg -- 15.72kb -> 5.24kb (66.7%)
/images/apps/siren-ios.jpg -- 19.56kb -> 6.51kb (66.69%)
/images/apps/atom.jpg -- 20.37kb -> 6.79kb (66.67%)
/images/apps/siren-android.jpg -- 26.27kb -> 8.78kb (66.58%)
/images/apps/falken.jpg -- 31.39kb -> 10.50kb (66.55%)
/images/apps/team-dashboard.jpg -- 24.67kb -> 8.33kb (66.21%)
/images/apps/nodegh.jpg -- 16.51kb -> 5.58kb (66.21%)
/images/apps/project-monitor.jpg -- 20.50kb -> 6.94kb (66.13%)
/images/apps/brackets.jpg -- 18.90kb -> 6.42kb (66.03%)
/images/apps/buildnotify.jpg -- 20.87kb -> 7.09kb (66%)
/images/apps/bickle.jpg -- 21.24kb -> 7.25kb (65.89%)
/images/apps/travalizit.jpg -- 25.66kb -> 8.76kb (65.87%)
/images/apps/node-travis-encrypt.jpg -- 30.32kb -> 10.37kb (65.8%)
/images/apps/ccmenu.jpg -- 25.69kb -> 8.81kb (65.7%)
/images/apps/travis-jr.jpg -- 22.98kb -> 7.89kb (65.69%)
/images/apps/cli.jpg -- 23.15kb -> 8.04kb (65.25%)
/images/apps/travis-lite.jpg -- 18.58kb -> 6.49kb (65.06%)
/images/apps/travis7.jpg -- 18.79kb -> 6.59kb (64.94%)
/images/apps/travis-web.jpg -- 20.34kb -> 7.68kb (62.26%)
/images/apps/vim.jpg -- 24.47kb -> 9.74kb (60.2%)
/images/admin_only_toggle.png -- 280.93kb -> 159.51kb (43.22%)
/images/debug_buttons.png -- 8.97kb -> 5.44kb (39.34%)
/images/autocancellation.png -- 74.98kb -> 46.54kb (37.92%)
/images/repository-build-status-image.png -- 120.92kb -> 75.36kb (37.68%)
/images/caches-item.png -- 28.33kb -> 18.62kb (34.27%)
/images/repository-build-status-modal-with-url.png -- 65.70kb -> 44.12kb (32.85%)
/images/common-build-problems/repository-requests-page.png -- 128.44kb -> 86.95kb (32.3%)
/images/github-app-page.png -- 85.92kb -> 62.38kb (27.4%)
/images/remove-log.png -- 9.33kb -> 7.52kb (19.47%)
/images/legacy-services-repo.png -- 74.87kb -> 60.58kb (19.08%)
/images/ui/what-infrastructure.png -- 48.00kb -> 38.86kb (19.04%)
/images/autocancellation-example.png -- 25.53kb -> 21.07kb (17.49%)
/images/add_Markdown_bade_github.jpg -- 87.90kb -> 73.33kb (16.57%)
/images/personal-token.jpg -- 72.75kb -> 60.88kb (16.32%)
/images/qos.001.jpg -- 73.30kb -> 62.12kb (15.26%)
/images/github-app-repo.png -- 49.62kb -> 42.67kb (14%)
/images/apps/chroma-feedback.jpg -- 7.48kb -> 6.58kb (12.01%)
/images/travis-and-pusher-in-love.jpeg -- 55.29kb -> 48.75kb (11.83%)
/images/apps/ci-dashboard.jpg -- 11.46kb -> 10.26kb (10.41%)
/images/cli-cache.png -- 24.70kb -> 22.18kb (10.21%)
/images/user-token.png -- 85.67kb -> 78.61kb (8.24%)
/images/Tim_Lucas_200px.jpg -- 25.63kb -> 23.88kb (6.84%)
/images/hiro-hs.png -- 36.84kb -> 35.33kb (4.12%)
/images/apps/emacs.jpg -- 10.93kb -> 10.70kb (2.05%)