Merge pull request #20 from nijikon/drop-support-for-ruby-2.2.6

drop support for ruby lower than 2.2.6

.travis.yml

@@ -2,13 +2,10 @@ language: ruby
 sudo: false
 
 rvm:
-  - 2.0.0
-  - 2.1.8
-  - 2.2.4
+  - 2.2.6
   - 2.3.0
   - ruby-head
-  - jruby
-  - jruby-9.0.4.0
+  - jruby-9.1.14.0
   - jruby-head
   - rbx-2
 

rbnacl-libsodium.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |spec|
   spec.extensions    = ['ext/rbnacl/extconf.rb']
 
   spec.add_runtime_dependency "rbnacl", ">= 3.0.1"
+  spec.required_ruby_version = '>= 2.2.6'
 
   spec.add_development_dependency "bundler", "~> 1.5"
   spec.add_development_dependency "rake", ">= 10"