add .travis.yml file
use a specific physical font for the error images

Relying on the default logical font leads to differences
on different platforms. Tested on:

Darwin 14.0.0 Darwin Kernel Version 14.0.0: root:xnu-2782.1.97~2/RELEASE_X86_64 x86_64
Linux 3.13.0-40-generic #69-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux

Some of the png tests were failing on the linux machine
before.
Merge pull request #23 from brharrington/png-font

Png font