Register elarm_summary_sup invoking gproc in the OTP way `{via,_,_}`...

... rather than invoking it directly.  gproc is kept hardcoded in the
code - it is not in a configuration parameter.

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