Ignore .bundle folder

This is the recommended way of doing it, at least according to Heroku:
https://devcenter.heroku.com/articles/bundler-configuration

I noticed this while following the instructions from the Fastlane docs,
running `bundle install --with deveploment`. See:
https://docs.fastlane.tools/plugins/plugins-troubleshooting/

Author: mokagio

.gitignore

@@ -1,6 +1,9 @@
 .DS_Store
 /tmp
 
+# Ruby / Bundler
+.bundle/
+
 # Fastlane
 fastlane/README.md
 fastlane/report.xml