docs(lib): only generate docs if on master branch or a tag commit

.travis.yml

@@ -37,6 +37,7 @@ after_success: |
     ls target/debug &&
     ./kcov-master/tmp/usr/local/bin/kcov --coveralls-id=$TRAVIS_JOB_ID --exclude-pattern=/.cargo target/kcov target/debug/hyper-* &&
     [ $TRAVIS_PULL_REQUEST = false ] &&
+    [ "$TRAVIS_TAG" != "" || $TRAVIS_BRANCH = master ] &&
     [ $TRAVIS_RUST_VERSION = stable ] &&
     cargo doc --no-deps &&
     ./.travis/docs.sh