Log Message:
Update ruby-prawn to 0.15.0

Full CHANGELOG is https://github.com/prawnpdf/prawn/wiki/CHANGELOG and
here is summary.

Prawn 0.15.0 ¡½ 2014.02.16

Summarized notes follow, but you can also check out the complete diff from
0.14.0 to 0.15.0.

This release is a stepping stone to next month's 1.0 release, and it involves
more internal restructuring and policy updates than it does new fixes or
features. However, there are a few minor fixes and improvements that shipped
with this code, and we may do one or two more maintenance releases of 0.15.x
before 1.0 ships in mid-March.

It should be safe to upgrade to 0.15 if you've been using 0.13 or 0.14. If
you're still using 0.12 or earlier, you should either invest the time to
upgrade those projects now, or treat them as legacy code
indefinitely. Extension authors are encouraged to try out 0.15, because it is
our last major release before 1.0, and is a good snapshot of where we
currently are with things.

* We've laid out a plan for post-1.0 API compatibility

* Prawn now officially now support Ruby 2.1.0

* A couple new gems have been extracted

* Grids can now be safely redefined

* Document#render no longer raises encoding errors on JRuby+Windows

* Table column width calculations now tolerate minor floating point rounding
  errors

* TTFunk has been upgraded to 1.1.0, which provides Unicode astral plane
  support in TTF files.

* Prawn once again has no third-party runtime dependencies!