Run apt-get update before the build on Travis CI
Install lapack before the build on Travis CI

The libsundials-serial-dev package (2.4.0-5) in the Ubuntu version
installed on Travis CI has a missing dependency on
liblapack-dev. Install it before the build. See:

 - http://packages.ubuntu.com/precise/libsundials-serial-dev
 - http://packages.ubuntu.com/precise/liblapack-dev