lock source-map at version 0.6.1

source-map@0.7.0 pulls in native dependencies like `fs` and `path` which
doesn't work for Screeps.

package.json

@@ -37,6 +37,6 @@
     "typescript": "^2.7.1"
   },
   "dependencies": {
-    "source-map": "^0.7.0"
+    "source-map": "~0.6.1"
   }
 }