Call hash.end() after piping file through it

Fixes the following error when using browserify:

```
node_modules/gulp-cachebust/index.js:34
    return hash.read().toString('hex').substr(0, this.checksumLength);
                       ^
TypeError: Cannot call method 'toString' of null
  at CacheBuster.getChecksum (/Users/vast/src/beta.shkshk.ru/node_modules/gulp-cachebust/index.js:34:24)
  at CacheBuster.getBustedPath (/Users/vast/src/beta.shkshk.ru/node_modules/gulp-cachebust/index.js:38:25)
```

gulp task:

```coffee
bundler = browserify(entries: ["./" + app_config.paths.main_javascript], extenstions: [".coffee"])

gulp.task "javascripts", ["clean:javascripts"], ->
  bundler.bundle()
    .pipe(source("application.js"))
    .pipe(cachebuster.resources())
    .pipe(gulp.dest(app_config.buildpaths.assets))

```