Attempt to fix appveyor builds
Attempt to fix appveyor builds

Summary:
Looks like the last successful run on appveyor was using PHP
7.0.9.  The current PHP release fails to install and breaks the build,
so let's try pinning to this last known good version.
Closes https://github.com/facebook/watchman/pull/398

Differential Revision: D4283477

Pulled By: wez

fbshipit-source-id: 77a33c6b5228ab7c8df7250cdfc931db551894fd

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