Remove duplicate entries from Gemfile

Deploy script complains:

```
Your Gemfile lists the gem airbrake (~> 3.1.12) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
Your Gemfile lists the gem newrelic_rpm (~> 3.6.2.96) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```