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.
Update the README file to point at travis, along with some more documentation.

Since travis now works, point at the build results.  Also put a short blurb
about PVS in the README file, along with a link to the homepage.