Remove trailing slash from HIPCHAT_APP var

When I first ran the install script, this happened:

```
$ ./install.sh

	Working on chat-osx.css
mv: rename /Applications/HipChat.app/Contents/Resources//chat-osx.css to /Applications/HipChat.app/Contents/Resources//chat-osx.css_20140919_0909: No such file or directory
```