Update function signature for get_bucket mock, set validate=True if boto.Version >= '2.25.0'

Versions of boto prior to 2.25.0 do an expensive enumeration to see if a
bucket exists. Instead of stripping out validation completely, disable
it only on older versions.
Move constant to mrjob.utils
Merge pull request #952 from benjolitz/get_bucket_validate_fix

Update function for get_bucket mock, set validate=True if boto.Version >= '2.25.0'