Simplify .travis.yml

Put /usr/racket/bin in PATH and use makefile.
Revert to `raco link rackjure`

The makefile `link` target does a `raco pkg install --link`, which isn't
what we want for Travis testing.
Actually revert to using `raco` directly for everything.

Some of the options used in makefile don't work with older versions of
Racket. The makefile is really intended for use developing on newer
versions of Racket.