Bump .version in mustache.js in git hook when version has changed

This changes the pre-commit hook that we've used for years to keep the
version value found in `package.json` in sync with the one in
`mustache.js`, to change `mustache.mjs` instead since that's where the
source code lives now.

hooks/pre-commit

@@ -78,7 +78,7 @@ class Bumper
 end
 
 bumper = Bumper.new([
-	Source.new('mustache.js', /mustache.version = '([\d\.]*)'/),
+	Source.new('mustache.mjs', /version: '([\d\.]*)'/),
 	Source.new('mustache.js.nuspec', /<version>([\d\.]*)<\/version>/),
 ])
 bumper.start