Bump rake-compiler-dock to pick up Ruby 2.6, drop older ones

With this version bump, the precompiled mysql2 gem for Windows supports
Ruby 2.2, 2.3, 2.4, 2.5, 2.6. Windows Ruby 2.0 and 2.1 are dropped.

Author: sodabrew

Gemfile

@@ -23,7 +23,7 @@ end
 
 group :development do
   gem 'pry'
-  gem 'rake-compiler-dock', '~> 0.6.0'
+  gem 'rake-compiler-dock', '~> 0.7.0'
 end
 
 platforms :rbx do