Use `$HOME` as the default download location

[nobeh]

Thanks for this small handy wrapper module around GJF. In the suggested Makefile:

get-run-google-java-format:
	@-(cd .run-google-java-format && git pull -q) || git clone -q https://github.com/plume-lib/run-google-java-format.git .run-google-java-format

I think maybe it's better to use $HOME as the default download location which makes it easier to use in different projects:

get-run-google-java-format:
	@-(cd ${HOME}/.run-google-java-format && git pull -q) || git clone -q https://github.com/plume-lib/run-google-java-format.git ${HOME}/.run-google-java-format