refs #355 rails renamed master to main

[luk4s]

related with #355
By default there is no master branch in Rails repositories

[luk4s]

Hmm, test fails because Rails dropped ruby 2.6 and lesser. (rails/rails@6487836)
So actual main requires 2.7 or 3.
So I add to travis rvm 2.7.2, for check master of activeresource agains main of rails.

To decid: "drop" support ruby < 2.7 or just remove it from .travis or just ignore?

[rafaelfranca]

We can remove both 2.5 and 2.7 from the travis config.