Install libatlas-base-dev for Blas package

The Go blas package requires libcblas to be present on the system.
Travis environments run Ubuntu Linux so we need to install
libatlas-base-dev.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Merge pull request #37 from penberg/travis-cblas

Install libatlas-base-dev for Blas package