Prevent issues with the rubygems-bundler gem

* If `rubygems-bundler` is installed, we couldn't run `thor version:use`
  because `rubygems-bundler` would basically attempt to run `bundle exec
  thor`, but bundler hadn't yet been bootstrapped.

Author: alindeman

Rakefile

@@ -25,6 +25,7 @@ def run_command(command, opts={})
       puts "-"*40
       begin
         Bundler.with_clean_env do
+          ENV['NOEXEC'] = "0" # prevent rubygems-bundler from interfering
           sh command
         end
       rescue Exception => e