Travis: change up RVM versions to work around RVM bug

See https://github.com/travis-ci/travis-ci/issues/2220

Also changed "2.1.1" to just "2.1", which is purported always to use the latest
Ruby 2.1.x.

.travis.yml

@@ -3,15 +3,12 @@ rvm:
   - "1.9.2"
   - "1.9.3"
   - "2.0.0"
-  - "2.1.0"
-  - "2.1.1"
+  # Forgo 2.1.0 until Travis has a satisfactory fix for
+  # https://github.com/travis-ci/travis-ci/issues/2220
+  - "2.1"  # always the latest 2.1.x
   - "jruby-19mode"
 script:
   bundle exec rake test
 branches:
   except:
     - "readme-edits"
-matrix:
-  allow_failures:
-    rvm: "2.1.0"
-  fast_finish: true