`raco pkg install plait` fails on Racket 7.9

[ZacharyEspiritu]

Running raco pkg install plait on an Ubuntu 18.04 system with Racket 7.9 installed results in the following error:

Resolving "plait" via https://download.racket-lang.org/releases/7.9/catalog/
  raco pkg install: cannot find package on catalogs
  package: plait

(The reason why we [CS173 at Brown] are running this on Ubuntu 18.04 + raco pkg command-line utilities is because we autograde our assignments on Gradescope VMs, which we configure via raco pkg.)

A working workaround for this issue is to run raco pkg install using the GitHub .git link for the Plait repository:

raco pkg install https://github.com/mflatt/plait.git