chore(gitignore): Add c9 to gitignore

Adding `.c9` to `.gitignore`.

`.c9` is from Cloud9 IDE, an editor in the browser. Partnered with Google to [connect Google App Engine apps](https://c9.io/blog/build-apps-for-google-cloud-platform-on-cloud9/). [Purchased by Amazon](https://c9.io/blog/great-news/) a month ago. The best way to code on a Chromebook.

Close angular#2782

packages/angular-cli/blueprints/ng2/files/gitignore

@@ -13,6 +13,7 @@
 /.vscode
 .project
 .classpath
+.c9/
 *.launch
 .settings/