(maint) Fix Travis failures for 2.1.0 and 1.8.7.

Working around RVM issues on Travis CI by explicitly specifying the
patch level for 1.8.7 so that REE isn't used.

Working around an install issue with 2.1.0 by specifying 2.1.1 instead
of 2.1.0.
Merge pull request #2610 from peterhuene/travis/fix

(maint) Fix Travis failures for 2.1.0 and 1.8.7.