Ensure no beginning slash in synced file paths

lib/index.js

@@ -137,7 +137,9 @@ export default class Now extends EventEmitter {
 
 function toRelative (path, base) {
   const fullBase = /\/$/.test(base) ? base + '/' : base;
-  return path.substr(fullBase.length);
+  const relative  path.substr(fullBase.length);
+  if (relative.startsWith('/')) return relative.substr(1);
+  return relative;
 }
 
 function responseError (res) {