Make sure Digest::lookup always pick the correct js file name.

[ylt6]

Fix #151

[gauravtiwari]

Looks like this won't be needed after #153 as it generates flat manifest file.

{
  "application.js": "/packs/application-61c89ec4d430372bfdbf.js",
  "application.js.map": "/packs/application-61c89ec4d430372bfdbf.js.map",
  "clock.png": "/packs/clock-fc31531de5cc3518c7c658d5b83faa72.png",
  "counter.css": "/packs/counter-11f770942f5b77a0442f.css",
  "counter.css.map": "/packs/counter-11f770942f5b77a0442f.css.map",
  "counter.js": "/packs/counter-0bd141f6d9360cf4a7f5.js",
  "counter.js.map": "/packs/counter-0bd141f6d9360cf4a7f5.js.map",
  "hello_react.css": "/packs/hello_react-11f770942f5b77a0442f.css",
  "hello_react.css.map": "/packs/hello_react-11f770942f5b77a0442f.css.map",
  "hello_react.js": "/packs/hello_react-447e04580529cf7a332b.js",
  "hello_react.js.map": "/packs/hello_react-447e04580529cf7a332b.js.map",
  "lacuna-webfont.eot": "/packs/lacuna-webfont-54c0d12c6ab265792f0d8cfd81e481e4.eot",
  "lacuna-webfont.svg": "/packs/lacuna-webfont-4291fc75d6240959b4abe83809c90350.svg",
  "lacuna-webfont.ttf": "/packs/lacuna-webfont-1acb63487c4a72f7fd2e545d2e0090e2.ttf",
  "lacuna-webfont.woff": "/packs/lacuna-webfont-0eb6add367ef5fc69c4569894525bd52.woff"
}