Simplify syntax to execute RMV

We use `:rvm` just as we use `:rake` in other places.

config/deploy.rb

@@ -63,7 +63,7 @@ end
 task :install_bundler_gem do
   on roles(:app) do
     within release_path do
-      execute "rvm use #{fetch(:rvm1_ruby_version)}; gem install bundler"
+      execute :rvm, fetch(:rvm1_ruby_version), "do", "gem install bundler"
     end
   end
 end