Update ruby-prawn package to 0.12.0.

Prawn 0.12.0:

We continue our trek to Prawn 1.0 with 0.12.0. This release fixes many bugs
and adds some new features. A full list of changes is available via `git log
0.11.1..0.12.0`.

Features / Enhancements

* Rewrite project README [Gregory Brown]
* Speed improvements to text kerning [Jan De Poorter]
* Snapshotting now preserves your bounding box [Brad Ediger]
* Allow Document#float to teleport across pages and return to starting page
  [Brad Ediger]
* number_pages now defaults to numbering every page [Matthew Rudy Jacobs]
* Your bounding box is now reset when starting a new page with a different
  size or layout [Brad Ediger]

Major Bugfixes

* Fix multi-page templates (#199) [Chase M. Gray / Jonathan Greenberg]
* Make tables work in stretchy bounding boxes (#235) [Jan De Poorter / Brad
  Ediger]
* Fix TTF font metrics when a TTF font has a nonzero width for newlines (#245)
  [daduke / Katsuya Hidaka / Brad Ediger]
* Fix kerning bug on line width calculations (#251) [vspan / Brad Ediger]
* Fix images in stretchy bounding boxes creating a new page (#241) [Brad Ediger]
* Fix character_spacing to work based on character count, not byte length
  (#253) [Katsuya Hidaka / Brad Ediger]
* Fix NoMethodError under $KCODE==“u” [Michael Klein]
* Fix extra page break created when spans were used at the top of a page
  (#255) [Sean Russell / Brad Ediger]
* Fix problem with table cell backgrounds overlapping other cell’s borders by
  drawing all backgrounds before any borders (#226) [Kenta Murata / Brad
  Ediger]
* Remove old examples that were duplicated by our new manual (Michaël Witrant)
  Tables: use an epsilon to compare floating-point values [Kenta Murata]

(taca)