Bump Travis Ruby version to 2.2.0
Bump Travis Ruby version to 2.2.0

OS X builds started to fail after the release of RubyGems 2.5.0, so this
should fix it.

jsyeo/rubinius,sferik/rubinius,sferik/rubinius,jsyeo/rubinius,ngpestelos/rubinius,kachick/rubinius,kachick/rubinius,sferik/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,sferik/rubinius,kachick/rubinius,ngpestelos/rubinius,sferik/rubinius,jsyeo/rubinius,jsyeo/rubinius,ngpestelos/rubinius,ngpestelos/rubinius,ngpestelos/rubinius,kachick/rubinius,jsyeo/rubinius,ngpestelos/rubinius,jsyeo/rubinius,sferik/rubinius,jsyeo/rubinius,ngpestelos/rubinius,sferik/rubinius