Add support for testing PVS against SBCL automatically using Travis CI.

Add initial support for running PVS against a few versions of sbcl using
Travis CI.  Currently tests 1.21, 1.1.18, and 1.0.58.  The current stable
versions for Mac OSX (1.1.8/1.1.6) have a bug that is fixed upstream in SBCL,
and are thus not tested.