Relax ActiveResource 5 dependency to 5.x (#809)

To ensure we're always testing against the latest 5.x release

Gemfile_ar51

@@ -2,4 +2,4 @@ source "https://rubygems.org"
 
 gemspec
 
-gem "activeresource", "5.1.0"
+gem "activeresource", "~> 5.1"