fix: make .watchmanconfig a valid JSON file

Fixes the same issue seen over in the Command-T repo:

    wincent/command-t@4d2569a

Current version of watchman doesn't like these to be zero-byte files,
instead requiring them to be valid JSON, otherwise it dies with:

    watchman::RootResolveError: failed to resolve root:
    unable to resolve root /home/wincent/code/wincent:
    failed to parse json from /home/wincent/code/wincent/.watchmanconfig:
    '[' or '{' expected near end of file

.watchmanconfig

@@ -0,0 +1 @@
+{}