When adapter.syncable is false, don't sync

Added back the line to skip attempting to run the migration when an adapter doesn't support it.  Happy to expand on this.

For our Sails CouchDB Adapter (https://github.com/codeswarm/sails-couchdb-orm) this flag completely breaks.  Should our adapter implement a safe migration? If it's not syncable, why should we have to do that?