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.
Add ~>% and ~>_
Add docs for ~>%, ~>_, etc.
Add ~>f and some~>f
Tweak docs