Add clean target to make file

[marckhouzam]

The installation of some tools has hard-code paths in them. For example:

$ head -1 .acceptance/.venv/bin/robot
#!/Users/marckhouzam/git/acceptance-testing/.acceptance/.venv/bin/python3.7

If I want to move the acceptance-testing repo to a new directory on my computer, the installed tools will no longer work due to these hard-coded paths. The simplest solution is to cleanup and re-install them.

This commit as a 'make clean' target to easily remove the entire .acceptance directory.