Make gst-switch build with Ubuntu 14.04 (trusty tahr) on Travis-CI

Fixes https://github.com/timvideos/gst-switch/issues/57 and https://github.com/timvideos/gst-switch/issues/58

Builds now take less than 10 minutes (normally about 2-5 minutes).

| Job  | Duration     |  Finished     |  cc | ENV                              | OS    |
| ---- | ------------ | ------------- | --- | -------------------------------- | ----- |
| 96.1 | 2 min 9 sec  | 5 minutes ago | gcc | TEST=python-api TYPE=unittest    | linux |
| 96.2 | 3 min 27 sec | 4 minutes ago | gcc | TEST=python-api TYPE=integration | linux |
| 96.3 | 4 min 6 sec  | 3 minutes ago | gcc | TEST=module TYPE=python          | linux |
| 96.4 | 3 min 33 sec | 4 minutes ago | gcc | TEST=module TYPE=c               | linux |