Update outdated bundler in gemspec

rails.gemspec

@@ -41,6 +41,6 @@
   s.add_dependency "actiontext",    version
   s.add_dependency "railties",      version
 
-  s.add_dependency "bundler",         ">= 1.3.0"
+  s.add_dependency "bundler",         ">= 1.15.0"
   s.add_dependency "sprockets-rails", ">= 2.0.0"
 end