Ignore the gradle-app.setting file.

I believe this file can be ignored, considering that it's only useful on the machine on which it was created (unless the other users have the same exact path), and it can easily be recreated by running the '--gui' option.

It really has no reason to be there.
Merge pull request #2601 from Deathsbreed/patch-1

Ignore the gradle-app.setting file.