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.

index.js

@@ -219,7 +219,7 @@ module.exports = Base = Class.extend({
 
   dropTable: function(tableName, options, callback) {
 
-    if (arguments.length < 3) {
+    if (arguments.length < 3 && typeof(options) === 'function') {
       callback = options;
       options = {};
     }

package.json

@@ -26,6 +26,6 @@
   },
   "homepage": "https://github.com/db-migrate/db-migrate-base",
   "dependencies": {
-    "bluebird": "^2.9.34"
+    "bluebird": "^3.1.1"
   }
 }