Simplify Travis-CI process

Travis-CI uses virtualenv to keep testing completely separate from the
system libraries which is often great but not so much for NIPAP. Our
tests include building debian packages from our source and then
installing those, to include the package build process in the testing
process. Since we install the debian packages it means we need to have
the necessary dependencies for those installed.

Since Travis-CI does not include system site packages in its virtualenv,
we installed everything *again* inside the virtualenv, including
dependencies which we installed using pip. This led to everything being
installed twice, leading to confusion and to unnecessary long test
times.

With this change, we tell Travis-CI virtualenv to use site packages and
skip the whole step of installing any packages via pip and instead use
everything from debian repositories. Test time should decrease to about
half.