bump fly
bump fly

[finishes #93710648]

Submodule src/github.com/concourse/fly 668117f..64448db:
  > strip trailing forward slashes from target

Signed-off-by: Toby Rumans <516e7840355d33b790e9dc5c111fe7c7c270a08f@pivotal.io>

holgero/concourse,zachgersh/concourse,cjcjameson/concourse,squeedee/concourse,jbayer/concourse,Amit-PivotalLabs/concourse,squeedee/concourse,squeedee/concourse,longnguyen11288/concourse,beni55/concourse,concourse/concourse,holgero/concourse,concourse/concourse,sedouard/concourse,concourse/concourse,jbayer/concourse,zachgersh/concourse,CloudCredo/concourse,sedouard/concourse,longnguyen11288/concourse,cjcjameson/concourse,concourse/concourse,longnguyen11288/concourse,CloudCredo/concourse,CloudCredo/concourse,beni55/concourse,sedouard/concourse,beni55/concourse,Amit-PivotalLabs/concourse,cjcjameson/concourse,zachgersh/concourse,holgero/concourse,Amit-PivotalLabs/concourse,jbayer/concourse,concourse/concourse