Use new db-migrate which allows to specify the db backend

We use only mysql, so updating this means a lot of unused db backends
like sqlite do not need to be built with gyp anymore.
Note that this version is not yet released as stable, but works fine for
us. The outstanding issues are not related to our use-case from what I
can tell.

Fixes #82

migrations/20170117170621-users-drop-showTutorial.js

@@ -1,5 +1,4 @@
-var dbm = global.dbm || require('db-migrate');
-var type = dbm.dataType;
+'use strict';
 
 exports.up = function(db, callback) {
     db.runSql('ALTER TABLE users DROP COLUMN showTutorial', function (error) {