Extend MiniRake::DSL module to prevent inheritance pollution

```include MiniRake::DSL``` on toplevel will pollute ancestor tree of every object.

```extend MiniRake::DSL``` will add those methods only to ```main``` object.

Rake does the same: https://github.com/jimweirich/rake/blob/master/lib/rake/dsl_definition.rb#L153

minirake

@@ -287,7 +287,7 @@ module MiniRake
 end
 
 Rake = MiniRake
-include MiniRake::DSL
+extend MiniRake::DSL
 
 
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