Install specific version of fglrx on Travis-CI

The current version that gets installed automatically is broken in the
sense that it does not work on a CPU in the absence of a GPU (e.g. see
https://travis-ci.org/ddemidov/vexcl/jobs/18265418#L276).

This installs version 2:8.960-0ubuntu1 which does work.
Merge pull request #50 from ddemidov/fix-travis-conf

Install specific version of fglrx on Travis-CI