Cut version 0.17.0

Some people refer to releasing a new version of a gem as "cutting"
a new version. This commit marks the release of overcommit 0.17.0,
and follows the pattern of commits from previous releases.

Change-Id: I9557ea85f409169d79c6e617cbf016b1ecb95a23
Reviewed-on: http://gerrit.causes.com/42091
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>