Reject sourcemap before sending it to uglify (#63)

* Reject sourcemap before sending it to uglify

At some point today, our builds started failing with a sourcemap error
from this lib. Running the tests here makes it replicable:

```
  ● allow to disable source maps

    DefaultsError: `sourcemap` is not a supported option

      at DefaultsError.get (eval at <anonymous> (node_modules/uglify-js/tools/node.js:20:1), <anonymous>:71:23)
```

This fixes the error by rejecting the option uglify is now rejecting
before the whole thing crashes.

* Use delete to mutate options object instead of a new copy

* Upgrade serialize-javascript

index.js

@@ -7,13 +7,17 @@ function uglify(userOptions = {}) {
     throw Error("sourceMap option is removed, use sourcemap instead");
   }
 
-  const minifierOptions = serialize(
-    Object.assign({}, userOptions, {
-      sourceMap: userOptions.sourcemap !== false,
-      sourcemap: undefined,
-      numWorkers: undefined
-    })
-  );
+  const normalizedOptions = Object.assign({}, userOptions, {
+    sourceMap: userOptions.sourcemap !== false
+  });
+
+  ["sourcemap"].forEach(key => {
+    if (normalizedOptions.hasOwnProperty(key)) {
+      delete normalizedOptions[key];
+    }
+  });
+
+  const minifierOptions = serialize(normalizedOptions);
 
   return {
     name: "uglify",

package.json

@@ -26,7 +26,7 @@
   "dependencies": {
     "@babel/code-frame": "^7.0.0",
     "jest-worker": "^24.0.0",
-    "serialize-javascript": "^1.6.1",
+    "serialize-javascript": "^1.9.0",
     "uglify-js": "^3.4.9"
   },
   "peerDependencies": {