Add rbx-3 to the build matrix

[thedrow]

No description provided.

[coveralls]

Coverage decreased (-97.7%) to 0.0% when pulling 2814a7e on thedrow:patch-1 into 2fe70d6 on guard:master.

[e2]

There's really no point in running RBX-2 and 3 at the same time. 3 is enough, even if it's broken.

[thedrow]

Got it.

[coveralls]

Coverage remained the same at 97.691% when pulling 2814a7e on thedrow:patch-1 into 2fe70d6 on guard:master.

[coveralls]

Coverage decreased (-0.02%) to 97.668% when pulling b252326 on thedrow:patch-1 into 2fe70d6 on guard:master.

[junaruga]

@rymai next PR to be merged could be this.
rbx-2 has not been available environment on Travis any more.
Current master branch
https://travis-ci.org/guard/listen/builds/387076637

I think you can do cherry-pick @thedrow's commit, if he has no time to rebase.

[rymai]

@thedrow @junaruga Thanks! Superseded by #446.