Add installation step to install Danger

.travis.yml

@@ -5,8 +5,9 @@ osx_image: xcode8.3
 
 install:
   - ./scripts/install_swiftlint.sh
+  - bundle install --path vendor/bundle
 
 script:
   - swiftlint
   - ./scripts/build.sh
-  - bundle exec danger
+  - bundle exec danger --verbose