Be more optimistic about ruby version

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-ruby '2.3.1'
+ruby '>=2.3'
 
 gem 'scraperwiki', :git => 'https://github.com/openaustralia/scraperwiki-ruby/', :branch => 'morph_defaults'
 gem 'petrify', :git => 'https://github.com/adrianshort/petrify/'