Improve the Travis configuration

Now it should be less heavy on Travis and more useful, since the rspec
will be ran on all Ruby implementations but mspec will be ran only on
latest Ruby.

.travis.yml

@@ -2,13 +2,34 @@ language: ruby
 
 cache: bundler
 
-rvm:
-  - 1.8.7
-  - 1.9.3
-  - 2.0.0
-  - 2.1.0
-  - rbx
-  - jruby
+matrix:
+  fast_finish: true
+
+  include:
+    - rvm: 2.1.0
+      env: RUN=default
+
+    - rvm: 1.8.7
+      env: RUN=rspec
+
+    - rvm: 1.9.3
+      env: RUN=rspec
+
+    - rvm: 2.0.0
+      env: RUN=rspec
+
+    - rvm: rbx
+      env: RUN=rspec
+
+    - rvm: jruby
+      env: RUN=rspec
+
+  allow_failures:
+    - rvm: 1.8.7
+    - rvm: 1.9.3
+    - rvm: 2.0.0
+    - rvm: rbx
+    - rvm: jruby
 
 before_install:
   - git submodule update --init
@@ -20,5 +41,8 @@ before_install:
   - gem update --system 2.1.11
   - gem --version
 
+script:
+  - "bundle exec rake $RUN"
+
 notifications:
   irc: "irc.freenode.org#opal"