Some fixes

[aecay]

• remove references to bintray in readme
  • remove/stub out the makefile target for bintray releases, pending a replacement (#64)
• make the check makefile target run the unit tests, as is standard
• make the setup target run go mod download
• fix the invocation of goimports in the makefile to avoid trying to run it before itʼs installed
• update go modules
  • fixes a (probably nonserious) security alert in the package dependencies
• fixes to the test suite
  • add an option to allow deprecated ssl cert behavior (needs a better fix which is #65 but in the meantime...)
  • add a nonzero shutdownGracePeriod to the server, otherwise the tests all fail locally for me (but not in travis -- must be a weird race, possibly introduced in my newer version of go)

[howardburgess]

Sorry @aecay I missed this too - I have disabled, but not removed, Bintray releases in #70 as an interim measure.