Drop support for ruby 2.0

It is no longer maintained upstream

.travis.yml

@@ -1,7 +1,6 @@
 language: ruby
 
 rvm:
-  - 2.0
   - 2.1
   - 2.2
   - ruby-head

History.md

@@ -3,6 +3,7 @@
 ## 4.0.0 / not yet released / branch "master"
 
 ### Changed
+* Ruby 2.0 support dropped (#832) [Dylan Thacker-Smith]
 * Add to_number Drop method to allow custom drops to work with number filters (#731)
 * Add strict_variables and strict_filters options to detect undefined references (#691)
 * Improve loop performance (#681) [Florian Weingarten]

liquid.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |s|
   s.license     = "MIT"
   # s.description = "A secure, non-evaling end user template engine with aesthetic markup."
 
+  s.required_ruby_version     = ">= 2.1.0"
   s.required_rubygems_version = ">= 1.3.7"
 
   s.test_files  = Dir.glob("{test}/**/*")