Add support for .mjs files

Right now these are served as application/octect-stream
The latest mime module with latest mime-db thought
would serve them as application/javascript.

This would enable direct .mjs access without needing
?module or runtime transpilations.

package.json

@@ -19,7 +19,7 @@
     "gunzip-maybe": "^1.4.0",
     "invariant": "^2.2.2",
     "isomorphic-fetch": "^2.2.1",
-    "mime": "^1.3.6",
+    "mime": "^1.4.0",
     "mkdirp": "^0.5.1",
     "morgan": "^1.8.1",
     "ndjson": "^1.5.0",