Use find-parent-dir to find .frogrc

Untested because I cannot figure out how to install find-parent-dir.
deps: Use the short form "find-parent-dir"

... since apparently samth already registered the package at
https://pkg.racket-lang.org/info/find-parent-dir
Use the "only-in" form of "require", for explicitness
* frog/frog.rkt (find-frog-root): never return #f
Add find-parent-dir pkg to .travis.yml