Update Travis build matrix to use CRuby on the appropriate Ruby versions

Why?

JRuby takes significantly longer to run the test suite; this ensures
that we use MRI instead of JRuby as the wrapper when running non-JRuby.

.travis.yml

@@ -12,6 +12,18 @@ install: "bundle install"
 script: "bundle exec rake"
 jdk:
   - openjdk6
+matrix:
+  exclude:
+    - rvm: 1.9.3
+      jdk: openjdk6
+    - rvm: 2.0.0
+      jdk: openjdk6
+    - rvm: 2.1.0
+      jdk: openjdk6
+    - rvm: 2.2.2
+      jdk: openjdk6
+    - rvm: rbx-2
+      jdk: openjdk6
 gemfile:
   - gemfiles/3.2.gemfile
   - gemfiles/4.0.gemfile