Ignore failures on JRuby and Rubinius

If tests pass on MRI but fail on JRuby or Rubinius, that indicates a
compatibility bug in these Ruby implementations, not a bug in this
library. Continue to run the tests so we can report and track these
issues but don't turn the entire build status red.

.travis.yml

@@ -45,9 +45,7 @@ matrix:
     - rvm: 1.8.7
       gemfile: gemfiles/sqlite3/4-0.gemfile
   allow_failures:
+    - rvm: rbx-18mode
+    - rvm: rbx-19mode
+    - rvm: jruby-18mode
     - rvm: jruby-19mode
-      gemfile: gemfiles/mysql/4-0.gemfile
-    - rvm: jruby-19mode
-      gemfile: gemfiles/postgresql/4-0.gemfile
-    - rvm: jruby-19mode
-      gemfile: gemfiles/sqlite3/4-0.gemfile