fix windows download link and linux install deps
fix windows download link and linux install deps

Summary:
Explicitly filter out pull requests from the possible set of downloads!

Refs: https://github.com/facebook/watchman/issues/583
Refs: https://github.com/facebook/watchman/pull/584

Also explicitly state the libssl-dev dep.
Closes https://github.com/facebook/watchman/issues/585

Reviewed By: chadaustin

Differential Revision: D7241584

fbshipit-source-id: 44f8d7058144b5064c41c650ba79573386be0047

nodakai/watchman,facebook/watchman,wez/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,wez/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,facebook/watchman