Bump src/collector

Bump cloudfoundry/collector:
  Dakota Bailey:
     Report aggregate app requests ( Travis Unknown )
  Jared Carroll:
     Router requests received metric ( Travis Success: https://travis-ci.org/cloudfoundry/collector/builds/18421346 )
  Phan Le:
     updated vcap_common and started reporting mem_bytes instead of mem from varz ( Travis Success: https://travis-ci.org/cloudfoundry/collector/builds/18363920 )