Build schema is always taken from the compiler's latest master

Only users of the latest version actually get correct autocompletions for their rescript.json, e.g. I now get 
"Property pinned-dependencies is not allowed." in a ReScript 11 project.

We should find some way for 
https://github.com/rescript-lang/rescript-vscode/blob/master/package.json#L54
to be aware of the version