Delete tcl, tk dependency with NOTK=1 on Ruby20, 21

ext/tk will not be built with NOTK=1. It is same as Ruby 18, 19.
Merge pull request #225 from shirosaki/notk

Delete tcl, tk dependency with NOTK=1 on Ruby20, 21
Update README with DevKit req. for Ruby 2.x

Both Ruby 2.0.x and 2.1.x are built using a different version of DevKit than the default one.

Modify README.txt to reflect this.

Closes #231