Enable TravisCI integration

TravisCI can build and test most of mono and provide timely feedback about regressions introduced on the main branch (and even before merge on pull requests).
Currently this configuration does not test all of mono, in particular for the profiles net_2_0, net_4_0 and net_4_5 only corlib is tested; additionally, mono/tests/bug-18026.exe often fails, so it cannot be trusted when checking for regressions (but apparently the other tests which are disabled from the CI=1 setting pass just fine).