Revert to old Travis build matrix

Reverts back to before d57c2ea and 898b9d2 were applied.

This is incredibly frustrating. Even after updating Rubygems in the
Travis build environment, we still see build failures when Travis
`bundle install`s with `--jobs=3`.

While the general consensus is that this should be fixed by running
`gem update --system`, that is clearly not the case. For now, I'm nixing
adding these builds in favor of a clean build.

Author: sds

.travis.yml

@@ -8,8 +8,7 @@ rvm:
   - 2.0
   - 2.1
   - 2.2
-  - 2.3.3
-  - 2.4.0
+  - 2.3.0
   - jruby-9.1.2.0
 
 matrix: