Relax Ruby version requirement (#1901)

.travis.yml

@@ -14,6 +14,7 @@ addons:
   postgresql: 9.4
 
 rvm:
+  - 2.3.4
   - 2.4.1
 
 services:

Gemfile

 # frozen_string_literal: true
 
 source 'https://rubygems.org'
-ruby '2.4.1'
+ruby '>= 2.3.0', '< 2.5.0'
 
 gem 'pkg-config'