Fix Travis Rubinius build
Added rubysl gems used by Prawn to the Gemfile
Merge pull request #604 from prawnpdf/travis-rbx

Travis rbx fix
Fix broken test
Remove AFM gem
Remove Ruby 1.8 specific code
Removed superfluous exteranal encoding specifier

"b" implies ASCII-8BIT external encoding.
Removed encoding method guards
Use encoding constants instead of encoding names
Merge pull request #605 from prawnpdf/ruby18-cleanup

Remove Ruby 1.8 specific code
Avoid setting y position unless modified in canvas

This is a clarification on the desired behavior of bounding boxes, which
essentially states that if the user does not modify the y
position in a bounding box that is either stretchy or
has its position held, the original y position will be restored.

The only realistic condition I can think of for this is
when doing vector drawings which do not actually modify the cursor
position, or possibly when doing conditional logic within a bounding
box call. However, this does fix the problem discovered in #523,
where calling canvas without modifying the y position ends
up putting the cursor at the absolute top of the page, outside
of the margins.

This is not an ideal fix, but it (mostly) preserves existing
behavior while providing sane behavior for an edge case.
Merge pull request #607 from prawnpdf/afm_bugs

Remove AFM gem
Mark #578 test as resolved
JPG images optimization

A few unnecessary reads can cost you a lot.

For a big JPEG (~9 MB) speed up is noticeable.

             user     system      total        real    memory
Before: 32.770000   0.260000  33.030000 ( 33.491774)  34.5 MB
After:  19.600000   0.050000  19.650000 ( 19.793003)  29.6 MB

Speed: **1.68x faster**
Memory: **14% less**

             user     system      total        real   memory
Before: 37.660000   0.200000  37.860000 ( 38.482895)  33.9 MB
After:  24.170000   0.060000  24.230000 ( 24.422936)  29.8 MB

Speed: **1.56x faster**
Memory: **12% less**

                user     system        total         real   memory
Before: 10487.800000 295.590000 10783.390000 (7131.061000)  528.3 MB (549.9 MB peak)
After:      7.350000   0.180000     7.530000 (   6.400000)  114.7 MB

Speed: **1,432x faster**
Memory: **78% less**

             user     system      total        real   memory
Before: 21.270153   0.239070  21.509223 ( 20.862641)  150.1 Mb
After:  16.131227   0.186280  16.317507 ( 15.169135)  143.4 MB

Speed: **1.32x faster**
Memory: **4% less**
Removed unused JPEG_APP_BLOCKS constant
Added option to ColumnBox to reset the top margin

This allows for page title text on the first page that is not repeated
on every page. The default is off, so it is completely backward
compatible. I also added a spec to verify the existing behavior
(which was not previously tested) and a spec to for the new option.
Merge pull request #610 from prawnpdf/jpg-optimization

Jpg optimization
Move pdf-core into its own gem
Add Ruby 2.1.0 to CI
Use require_relative to prevent encoding.rb from being loaded twice