Travis: Collecting the config

- comment
  - tighter matrix
  - pull out development dependencies into the Gemfile

Gemfile

@@ -2,14 +2,20 @@ source "https://rubygems.org"
 
 gemspec
 
-group :test do
+group :development do
+end
+
+group :development, :test do
   gem "rake"
   gem "bundler"
-  gem "rubocop"
-  gem "overcommit"
+  gem "overcommit", ">= 0.31"
+  gem "rubocop", ">= 0.43"
+end
+
+group :test do
   gem "coveralls", "~>0.8", require: false
   gem "simplecov", "~>0.10", require: false
   gem "codeclimate-test-reporter", "~>0.4"
-  # JSON 2.0.1 is ruby 2.0+
-  gem "json", "< 2.0"
+  gem "json"
+  gem "rspec", "< 4"
 end