Update Ruby versions in .travis.yml and add 2.4.
Update Ruby versions in .travis.yml and add 2.4.

It used to be the case that RVM (and thus Travis) would let you use,
say, "2.0" to refer to the latest 2.0.x release. Patch versions now need
to be given explicitly. See github/rvm/rvm 3512.

kschiess/parslet,kschiess/parslet,NigelThorne/parslet,NigelThorne/parslet,NigelThorne/parslet,kschiess/parslet