Run rubocop on Travis.

Author: anolson

.travis.yml

@@ -7,3 +7,6 @@ rvm:
 before_install:
   - gem update --system
   - gem install bundler
+script:
+  - bundle exec rake test
+  - bundle exec rake rubocop

git_diff.gemspec

@@ -19,4 +19,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 2.0"
   spec.add_development_dependency "minitest"
   spec.add_development_dependency "rake"
+  spec.add_development_dependency "rubocop", "~> 0.74.0"
 end