Temporarily disable Mirage2 build on Travis CI

Random "Killed" messages have reappeared from Travis ever since I merged #637.  According to Travis CI, this is probably a Memory-related issue: 
http://docs.travis-ci.com/user/common-build-problems/#My-build-script-is-killed-without-any-error

This PR simply disables the Mirage2 build on Travis CI for now, so that we can have time to investigate how best to re-enable it without causing Travis CI instability.