revert sourcemap version

because newer versions contain Rust code (in WebASM) and it is non-trivial to upgrade

Author: philschatz

package.json

@@ -14,7 +14,7 @@
     "phantomjs-prebuilt": "2.1.16",
     "puppeteer": "^1.5.0",
     "sizzle": "2.3.3",
-    "source-map": "0.7.3",
+    "source-map": "0.5.6",
     "standard": "^11.0.1"
   },
   "bin": {