correct git ref matching in Gemfile

Author: kares

Gemfile

@@ -3,7 +3,7 @@ source "https://rubygems.org"
 if version = ENV['AR_VERSION']
   if version.index('/') && ::File.exist?(version)
     gem 'activerecord', :path => version
-  elsif version =~ /[0-9abcdef]*/
+  elsif version =~ /^[0-9abcdef]+$/
     gem 'activerecord', :github => 'rails/rails', :ref => version
   elsif version.index('.').nil?
     gem 'activerecord', :github => 'rails/rails', :branch => version