Builds tend to fail due to the 10 min limit for stalled builds.
Most likely because the download is done silently and takes more than
10 min depending on the Travis connection.
Now it test 6 builds:
* Ubuntu 14.04 - x11 (g++)
* Ubuntu 14.04 - x11 (clang++)
* Ubuntu 14.04 - windows (via MinGW)
* OS X - osx (g++)
* OS X - osx (clang++)
* OS X - android
* OS X - iphone
Instead of just g++ on Ubuntu 12.04...