Update build.sbt

mysql-connector-java 5.1.18 cause exception.

```
play.api.Application$$anon$1: Execution exception[[MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'OPTION SQL_SELECT_LIMIT=DEFAULT' at line 1]]

```

And change to the latest version can solve this problem.