Fix eunit compile failure on Erlang 17+ (hamcrest)

[marco-m]

This fixes the error:

make test
...
==> meck (eunit)
...
meck/deps/hamcrest/include/hamcrest_internal.hrl:33: type set() undefined
meck/deps/hamcrest/include/hamcrest_internal.hrl:34: type gb_set() undefined
Compiling test/meck_tests.erl failed:
ERROR: eunit failed while processing meck: rebar_abort

and finally lets eunit run (and fail, at least with Erlang 18.3), but at least travis will go one step ahead :-)

[eproxus]

Thanks so much! I had this on my todo list for quite a while now, but didn't realise the fix could be that easy! 😄 The build just failed because of a random Travis CI crash (happens surprisingly often with Erlang builds for some reason). I restarted and now it's fine, all green. ✅

[marco-m]

:-)