Use travis-support/master

This makes the worker use current travis-support/master.

The log format very slightly changes. Before:

Before:
```
2014-10-08T13:35:19.104000+00:00 5613 DEBUG TID=ees [linux-1:worker:instance] done: prepare
```

After:
```
2014-10-08T16:20:34.189000+00:00 D PID=23938 TID=9444 [linux-1:worker:instance] done: prepare
```