Use 'trusty' image for travis now that it's public

Travis CI team has recently published beta image of Trusty for testing
[1]. Switch to that image for CI builds in a non-hacky way.

[1] http://blog.travis-ci.com/2015-10-14-opening-up-ubuntu-trusty-beta/

.travis.yml

@@ -28,6 +28,7 @@ env:
     - OPENRCT2_CMAKE_OPTS="-DDISABLE_NETWORK=ON -DDISABLE_HTTP_TWITCH=ON" TARGET=docker32
 
 sudo: required
+dist: trusty
 
 services:
     - docker