patch boto 2.10.0 thru 2.15.0 to fix #778

(tested this with Python 2.6.7, boto versions 2.9.9, 2.14.0, and 2.16.0,
and AWS regions us-west-1 and '')
Merge branch 'master' of github.com:Yelp/mrjob into boto_hmac_v4_patch
Merge pull request #823 from davidmarin/boto_hmac_v4_patch

patch boto 2.10.0 thru 2.15.0 to fix #778