fix(config): don't set a default value for db arg, as it breaks local installs

closes #323
- remove default yargs value to fix local installs

Author: acburdine

lib/commands/config/advanced.js

@@ -69,7 +69,6 @@ module.exports = {
     db: {
         description: 'Type of database Ghost should use E.g. mysql or sqlite3',
         validate: value => includes(['sqlite3', 'mysql'], value),
-        default: 'mysql',
         configPath: 'database.client',
         type: 'string',
         group: 'Database Options:'