fix(api): drop table overwrite options on promises

If drop table was called as promise, it was not explicitly checked if options
is a function and was used and set as callback.

Also upgraded bluebird dependency.

Author: wzrdtales

package.json

@@ -29,7 +29,7 @@
   },
   "homepage": "https://github.com/db-migrate/pg",
   "dependencies": {
-    "bluebird": "^2.9.34",
+    "bluebird": "^3.1.1",
     "db-migrate-base": "^1.2.0",
     "sqlite3": "^3.0.10"
   }