Repeatedly attempt to merge a pull request...

...until GitHub catches up.

```
[info]   java.io.FileNotFoundException: https://api.github.com/repos/prout-travis-bot/prout-test-1415313488465/pulls/1/merge
[info]   at com.squareup.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:228)
[info]   at com.squareup.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
[info]   at com.squareup.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:25)
[info]   at org.kohsuke.github.Requester.parse(Requester.java:383)
[info]   at org.kohsuke.github.Requester._to(Requester.java:185)
[info]   at org.kohsuke.github.Requester.to(Requester.java:160)
[info]   at org.kohsuke.github.Requester.to(Requester.java:148)
[info]   at org.kohsuke.github.GHPullRequest.merge(GHPullRequest.java:226)
[info]   at lib.Helpers$class.mergePullRequestIn(Helpers.scala:89)
[info]   at FunctionalSpec.mergePullRequestIn(FunctionalSpec.scala:5)
```