Bump src/gnatsd

Bump apcera/gnatsd:
  Derek Collison:
     copyright hygiene ( Travis Success: https://travis-ci.org/apcera/gnatsd/builds/18390627 )
     Fixes issues #21 and #22 ( Travis Success: https://travis-ci.org/apcera/gnatsd/builds/18390377 )
     Proper link to Fowler algorithm, fixes #23 ( Travis Success: https://travis-ci.org/apcera/gnatsd/builds/18102226 )