Remove ruby versions we don't care on CI

Author: ixti

.travis.yml

@@ -12,21 +12,14 @@ env:
     - JRUBY_OPTS="$JRUBY_OPTS --debug"
 
 rvm:
+  - jruby-9.1.7.0
   - 2.0.0
   - 2.1
   - 2.2
-  - 2.3.0
-  - 2.3.1
-  - 2.3.2
   - 2.3.3
   - 2.4.0
-  - ruby-head
-  - jruby-9.1.6.0
-  - jruby-9.1.7.0
 
 matrix:
-  allow_failures:
-    - rvm: ruby-head
   fast_finish: true
 
 branches: