(QE-428) bundle install fails on Ruby 1.8.7 because of new mime-types 2.0 dependency requiring 1.9.2

- add entry to gemspec to use mime-types 1.25 with ruby 1.8.7
Merge pull request #45 from anodelman/187-mimetypes

(QE-428) bundle install fails on Ruby 1.8.7 because of new mime-types 2.0 dependency requiring 1.9.2