Relax PATCH level in .travis.yml for ruby versions.

Ruby is now using SEMVER. Relaxing to 2.0 and 2.1 gives them the freedom to
release new patches and for us to take advantage of them without modifying
this file.
https://www.ruby-lang.org/en/news/2013/12/21/semantic-versioning-after-2-1-0/

.travis.yml

@@ -4,8 +4,8 @@ install:
 - script/rebund download
 - travis_retry bundle install --path vendor/bundle
 rvm:
-- 2.1.2
-- 2.0.0
+- 2.1
+- 2.0
 - 1.9.3
 script: script/cibuild
 after_script: