Drop support for Ruby 2.0

Support for Ruby 2.0 ended in February 2016 [1].

While we have continued to support it anyway, our Travis tests are now
failing due to some gems explicitly requiring Ruby >= 2.1.

[1] https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/

Author: sds

.travis.yml

@@ -5,7 +5,6 @@ sudo: false
 cache: bundler
 
 rvm:
-  - 2.0
   - 2.1
   - 2.2
   - 2.3.0

README.md

@@ -55,7 +55,7 @@ writing any Ruby code.
 
 This project aims to support the following Ruby runtimes on both \*nix and Windows:
 
-* MRI 2.x
+* MRI 2.1+
 * JRuby 9+
 
 ## Limitations

overcommit.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
                             Dir['libexec/**/*'] +
                             Dir['template-dir/**/*']
 
-  s.required_ruby_version = '>= 2'
+  s.required_ruby_version = '>= 2.1'
 
   s.add_dependency             'childprocess', '~> 0.5.8'
   s.add_dependency             'iniparse', '~> 1.4'