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

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

https://github.com/carrierwaveuploader/carrierwave/pull/1109