amend Rake 0.9 support

[quix]

The original commit for Rake 0.9 support (6c3123a) was problematic: the top level include Rake::DSL pollutes Object with the methods 'task', 'file', etc., which is what Rake 0.9 intended to correct. Also, requiring a private Rake header (dsl_definition.rb) is asking for trouble.

[Update] This pull request passes all working specs.

The relevant section of the Rake's ChangeLog:

* *Incompatible* *change*: Rake DSL commands ('task', 'file', etc.) are
  no longer private methods in Object. If you need to call 'task :xzy' inside
  your class, include Rake::DSL into the class. The DSL is still available at
  the top level scope (via the top level object which extends Rake::DSL).

[quix]

Updated above description -- possible spec failure previously mentioned was unrelated.

[indirect]

definitely better, thanks!