Bumping minimum Ruby version.

Author: cgore

btce.gemspec

@@ -37,7 +37,7 @@
 
 Gem::Specification.new do |s|
   s.name = 'btce'
-  s.version = '0.5.3'
+  s.version = '0.5.4'
   s.date = '2016-08-18'
   s.summary = "A simple library to interface with the API for btc-e.com in Ruby."
   s.description = "A simple library to interface with the API for btc-e.com in Ruby."
@@ -56,5 +56,6 @@ Gem::Specification.new do |s|
   s.email = 'cgore@cgore.com'
   s.files = `git ls-files lib/`.split($/)
   s.homepage = 'https://github.com/cgore/ruby-btce'
+  s.required_ruby_version = ">= 2.2.2"
   s.add_dependency 'monkey-patch', "~> 0.1.0"
 end