Update ruby version for RVM

Under 1.9.2, `bundle install` fails with the following error:

Gem::InstallError: activesupport requires Ruby version >= 1.9.3.
An error occurred while installing activesupport (4.0.0), and Bundler 
cannot continue. Make sure that `gem install activesupport -v '4.0.0'`
succeeds before bundling.

.rvmrc

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-ruby_string="ruby-1.9.2-p290"
+ruby_string="ruby-1.9.3"
 gemset_name="wicked"
 
 if rvm list strings | grep -q "${ruby_string}" ; then