Update ruby-prawn to 2.0.1.

* pkgsrc change: Allow this pacakge build on Ruby 2.2.

PrawnPDF 2.0.1 -- 2015-03-23

Fix regression in draw_text() with rotation

Due to missing tests, a typo snuck into the draw_text() method in PDF::Core,
preventing it from working properly when called with the :rotate option.

This issue has been resolved, and a test has been added to Prawn's test
suite. Speaking more generally, we need to improve the condition of the
tests for PDF::Core, and make a clear separation between Prawn's test suite
and PDF::Core's tests. Currently there are lots of little gaps that can lead
to this sort of problem.

[Robert S. Gerus, pdf-core#15]

(taca)