travis: update before install

It may fix the following error:

    Failed to fetch http://us.archive.ubuntu.com/ubuntu/pool/universe/m/mysql-5.5/mysql-testsuite-5.5_5.5.31-0ubuntu0.12.04.2_amd64.deb 404 Not Found [IP: 2001:67c:1562::13 80]

    Failed to fetch http://us.archive.ubuntu.com/ubuntu/pool/universe/m/mysql-5.5/mysql-testsuite_5.5.31-0ubuntu0.12.04.2_all.deb 404 Not Found [IP: 2001:67c:1562::13 80]

    E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?