Allow ActiveSupport >4.0.0

Ruby 2.4.0 does not work with version 1.8.3 of the JSON gem which was
required by ActiveSupport 4.x. Allowing use of ActiveSupport 5.x in turn
allows use of JSON 2.x which is compatible with 2.4.0

ruby/json#303

terrestrial.gemspec

@@ -26,6 +26,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "pg", "~> 0.17.1"
 
   spec.add_dependency "sequel", "~> 4.16"
-  spec.add_dependency "activesupport", "~> 4.0"
+  spec.add_dependency "activesupport", "> 4.0"
   spec.add_dependency "fetchable", "~> 1.0"
 end