Get the deps before compiling
Add PropEr as dependency and only test erlmqtt

Running `rebar eunit` will recursively test dependencies, which in
getopt's case fails because of (apparently) rebar including getopt
itself. https://github.com/jcomellas/getopt/issues/16. Gross. In the
meantime, I'll just run erlmqtt's tests only.

Since I'm using rebar dependencies now, I may as well add one for
PropEr, even though it's only used for testing. Maybe one day rebar
will differentiate between compile-time and run-time
dependencies. (https://github.com/rebar/rebar/issues/206)