(maint) Add Travis CI Support

Without this patch facter has Travis CI configuration files, but they don't
seem to completely specify the dependency versions and the build matrix.  This
patch addresses the problem by putting the dependency information in the
conventional Gemfile location.

This patch should coincide with enabling Travis CI support for pull requests.
A build status image is also included in the project README.  Finally, this
patch also removes the Gemfile.lock since this shouldn't be included in the
repository as Facter is more of application than it is a library.  This
reverses the decision to include it, which was my mistake, in 0b49eae and
(#15464).