Adding version file

Follow the standard structure of having a version.rb file

lib/liquid.rb

@@ -45,6 +45,7 @@ module Liquid
   VariableParser              = /\[[^\]]+\]|#{VariableSegment}+\??/o
 end
 
+require "liquid/version"
 require 'liquid/drop'
 require 'liquid/extensions'
 require 'liquid/errors'

lib/liquid/version.rb

@@ -0,0 +1,4 @@
+# encoding: utf-8
+module Liquid
+  VERSION = "2.5.0"
+end