Require Ruby 2.1+ for GC::INTERNAL_CONSTANTS

.travis.yml

@@ -1,5 +1,5 @@
 rvm:
-  - 2.1.4
+  - 2.1.7
 script:
   - rspec spec
 notifications:

README.md

@@ -21,7 +21,7 @@ Note that 3.0 is not backwards compatible with 2.x, so upgrade carefully! In par
 
 ## Installation
 
-Requires Ruby 2.0+
+Requires Ruby 2.1+. Ruby 2.3+ is recommended.
 
 ```bash
 gem install twitter_ebooks

twitter_ebooks.gemspec

@@ -2,6 +2,8 @@
 require File.expand_path('../lib/twitter_ebooks/version', __FILE__)
 
 Gem::Specification.new do |gem|
+  gem.required_ruby_version = '~> 2.1'
+
   gem.authors       = ["Jaiden Mispy"]
   gem.email         = ["^_^@mispy.me"]
   gem.description   = %q{Markov chains for all your friends~}