Remove rbx-2.5.8 and rbx-3.70 from Travis
Remove rbx-2.5.8 and rbx-3.70 from Travis

They are failing and we are ignoring them anyway. They also make
Travis to take longer to report the results.

Closes https://github.com/rantly-rb/rantly/issues/38

Ana06/rantly,abargnesi/rantly