Support Rails 4 on the master branch

Given that's where we do gem releases from anyway

So... what's the purpose of the Rails 4 branch?

.travis.yml

@@ -3,6 +3,9 @@ rvm:
   - 2.0.0
 
 env:
+-   RAILS=4-0-stable DB=sqlite
+  - RAILS=4-0-stable DB=mysql
+  - RAILS=4-0-stable DB=postgres
   - RAILS=3-2-stable DB=sqlite
   - RAILS=3-2-stable DB=mysql
   - RAILS=3-2-stable DB=postgres