Generalize hardcoded OSX_DEPLOYMENT_TARGET

The OSX_DEPLOYMENT_TARGET in the osx.yaml base file is now set to the
current version of OS X on the host machine.  Note that this change to
the prologue script will trigger a full stack rebuild in hashstack.
Merge pull request #71 from hashdist/general_osx_deployment

Generalize hardcoded OSX_DEPLOYMENT_TARGET