resolve test failure on travis-ci precise64 with ruby 1.8.7, rmagick 2.13.2, imagemagick 6.6.9-7 2012-08-17 Q16
Merge pull request #1115 from woodwardjd/maintain_file_conversion_across_processing

resolve test failure on travis-ci precise64 with ruby 1.8.7, rmagick 2.13.2, imagemagick 6.6.9-7 2012-08-17 Q16