Fix invocation of subscribe

This is the easiest fix.  A more complex change could be introduced
for remote subscription probably.

This commit fixes the following Dialyzer warning:
elarm_summary.erl:110: Call to missing or unexported function elarm_server:subscribe/2
Add minimal test re subscribe_summary
Merge pull request #23 from lucafavatella/add-subscribe_summary-test

Add minimal subscribe_summary test
Register elarm_summary_sup invoking gproc in the OTP way `{via,_,_}`...

... rather than invoking it directly.

Support for alternative registry was added in OTP behaviours by commit
b042673, that was first included in R15B01.

Ref: https://github.com/esl/elarm/issues/19