Don't call SyntasticCheck if not syntastic checker

If the user forced YCM to not register itself as the Syntastic checker, we
should not be calling SyntasticCheck since that would slow down everything.

Fixes #416

autoload/youcompleteme.vim

@@ -380,7 +380,8 @@ endfunction
 function! s:UpdateDiagnosticNotifications()
   if get( g:, 'loaded_syntastic_plugin', 0 ) &&
         \ pyeval( 'ycm_state.NativeFiletypeCompletionUsable()' ) &&
-        \ pyeval( 'ycm_state.DiagnosticsForCurrentFileReady()' )
+        \ pyeval( 'ycm_state.DiagnosticsForCurrentFileReady()' ) &&
+        \ g:ycm_register_as_syntastic_checker
     SyntasticCheck
   endif
 endfunction