Replace mysql npm module with mysql2

Our DB recently changed which auth mechanisms were allowed. We disabled
the native password option (the only one that the mysql npm module
supports). Thankfully, the mysql2 module is way more popular now and
supports the auth we want and the api is nearly identical.

bin/migrate

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 
 var config = require('../lib/config-loader');
-var mysql = require('mysql'),
+var mysql = require('mysql2'),
     fs = require('fs');
 
 var db = mysql.createConnection({

lib/db.js

@@ -1,6 +1,5 @@
-var mysql = require("mysql"),
-  config = require("./config-loader"),
-  Promise = require("bluebird");
+var mysql = require("mysql2/promise"),
+   config = require("./config-loader");
 
 const pool = mysql.createPool({
   host: config.mysql.host,
@@ -10,6 +9,4 @@ const pool = mysql.createPool({
   charset: "utf8mb4",
 });
 
-pool.query = Promise.promisify(pool.query);
-
 module.exports = pool;