More correct requirement specifier

Technically, if the rails team released a new "5.0.1.zeitgeist"
prerelease version and rubygems behavior was fixed according to
rubygems/rubygems#2597, this requirement would
no longer do what it's supposed to do.

This will clearly not happen, but I wanted to raise awareness of this
xD.

rails-controller-testing.gemspec

@@ -18,11 +18,11 @@ Gem::Specification.new do |s|
 
   s.required_ruby_version = '>= 2.2.2'
 
-  s.add_dependency "actionpack", ">= 5.0.1.x"
-  s.add_dependency "actionview", ">= 5.0.1.x"
-  s.add_dependency "activesupport", ">= 5.0.1.x"
+  s.add_dependency "actionpack", ">= 5.0.1.rc1"
+  s.add_dependency "actionview", ">= 5.0.1.rc1"
+  s.add_dependency "activesupport", ">= 5.0.1.rc1"
 
-  s.add_development_dependency "railties", "> 5.0.1.x"
+  s.add_development_dependency "railties", ">= 5.0.1.rc1"
 
   if defined?(JRUBY_VERSION)
     s.add_development_dependency "jdbc-sqlite3"