fix(major): use mysql2 library if present

fixes TryGhost/Toolbox#306

- this fixes an issue whereby newer versions of Ghost ship with mysql2
  and not mysql, but to maintain compatibility, the config uses `mysql`
- this means knex would attempt to use the `mysql` lib, but the library
  wouldn't exist
- this commit adds a check to see if `mysql2` exists and override the
  client we connect with if so

lib/tasks/major-update/data.js

@@ -15,15 +15,27 @@ module.exports = async function getData({dir, database, versionFolder, version})
 
     const knexPath = path.resolve(dir, versionFolder, 'node_modules', 'knex');
     const gscanPath = path.resolve(dir, versionFolder, 'node_modules', 'gscan');
+    const mysql2Path = path.resolve(dir, versionFolder, 'node_modules', 'mysql2');
 
     const knex = require(knexPath);
     const gscan = require(gscanPath);
 
+    try {
+        // If we're using MySQL, check to see if the `mysql2` lib exists because we
+        // need to change the `client` to maintain backwards compatibility with config
+        if (database.client === 'mysql') {
+            require(mysql2Path);
+            database.client = 'mysql2';
+        }
+    } catch (err) {
+        // `mysql2` doesn't exist, so we can stay with `mysql`
+    }
+
     const connection = knex({...database, useNullAsDefault: true});
     const query = async (sql) => {
         const response = await connection.raw(sql);
 
-        if (database.client === 'mysql') {
+        if (['mysql', 'mysql2'].includes(database.client)) {
             return response[0][0];
         }