Reduce size of Travis matrix

We excluded almost as much as we care about and adding COVERAGE created
a number of invalid combinations so this changes the matrix to include
the workable combinations of Ruby version and Gemfile.

COVERAGE is only on Ruby 1.9.3 whilst I confirm this is working

.travis.yml

@@ -1,11 +1,5 @@
 language: ruby
 
-gemfile:
-  - Gemfile
-  - Gemfile.1.8.7
-
-env: COVERAGE=true
-
 rvm:
   - 1.8.7
   - 1.9.2
@@ -19,23 +13,24 @@ rvm:
 script: bundle exec rake travis
 
 matrix:
-  exclude:
+  include:
     - rvm: 1.8.7
-      gemfile: Gemfile
-    - rvm: 1.9.2
       gemfile: Gemfile.1.8.7
+    - rvm: 1.9.2
+      gemfile: Gemfile
     - rvm: 1.9.3
-      gemfile: Gemfile.1.8.7
+      gemfile: Gemfile
+      env: COVERAGE=true
     - rvm: 2.0.0
-      gemfile: Gemfile.1.8.7
+      gemfile: Gemfile
     - rvm: 2.1.0
-      gemfile: Gemfile.1.8.7
-    - rvm: jruby-18mode
       gemfile: Gemfile
-    - rvm: jruby-19mode
+    - rvm: jruby-18mode
       gemfile: Gemfile.1.8.7
+    - rvm: jruby-19mode
+      gemfile: Gemfile
     - rvm: jruby-head
-      gemfile: Gemfile.1.8.7
+      gemfile: Gemfile
 
   allow_failures:
     - rvm: jruby-head