fix bin exclude in .gitignore
fix bin exclude in .gitignore

Summary: `bin/` contains build artifacts and should be excluded, but `packages/flow-dev-tools/bin/tool` should not be.

Reviewed By: dsainati1

Differential Revision: D24480142

fbshipit-source-id: a38b536343aa309a6636d34cf93ee2518c5e1cef

mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow