Switched symlink to symlinkSync

- we don't provide a callback for this so we want it synchronous
  but `symlink` is async. Switching to `symlinkSync` should now fix the
  flaky tests

ghost/zip/test/zip.test.js

@@ -32,7 +32,7 @@ describe('Compress and Extract should be opposite functions', function () {
     });
 
     it('ensure symlinks work', function (done) {
-        fs.symlink(folderToSymlink, symlinkPath);
+        fs.symlinkSync(folderToSymlink, symlinkPath);
 
         let originalHash;