Update RubyGems prior to installing bundle

By enabled Danger, we enable the installation of Rubocop on the CI
environment. Rubocop depends on the "rainbow" library which has a build
error on certain versions of RubyGems. This has been fixed in newer
versions of RubyGems, so let's just update RubyGems prior to trying to
install anything.

See: ku1ik/rainbow#44
See: rubygems/rubygems#1819

.travis.yml

@@ -10,7 +10,10 @@ rvm:
 script: bundle exec rspec
 sudo: false
 
-before_install: gem install bundler -v 1.11.2
+before_install:
+  - gem update --system --no-doc
+  - gem install bundler -v 1.14.1
+
 bundler_args: --without development
 
 matrix: