Update .ackrc for ack 2.12

Summary:
Just installed ack, got this when doing anything:
```
Unknown option: invert-file-match
Option '-G' is not valid in ack 2
```

Test Plan: ack pattern works

Reviewers: marcos

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D9038
Update .ackrc for ack 2.12

Summary:
Just installed ack, got this when doing anything:
```
Unknown option: invert-file-match
Option '-G' is not valid in ack 2
```

Test Plan: ack pattern works

Reviewers: marcos

Reviewed By: marcos

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D9038
Merge branch 'master' of http://github.com/Khan/khan-dotfiles