Merge pull request #61 from agworld/fix_subtract_class

Fix failing case of subtraction losing Units class
Test fix for 1.8.7
Fixing :to_date for 1.8 rubies, and making sure it detects if method exists in 1.8
remove gemnasium badge
Merge pull request #63 from drasch/to_date

fixes an issue with to_date on 1.8.7.  

Please be aware that I no longer officially support ruby 1.8.7.  
Thanks for the patch, David.  Good to hear from you.
Merge remote-tracking branch 'upstream/master'