Update image_optim from 0.14.0 -> 0.15.0

The latest version of the `image_optim` gem broke Overcommit's
`ImageOptim` pre-commit hook due to a change in the name of the
`BinNotFoundError` exception. Fix it and change the version constraint
in the gemspec.

Change-Id: Ie938de988887b2794115c35c45d654c7f7d9d678
Reviewed-on: http://gerrit.causes.com/42208
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>