Cache isn't invalidated when file is removed

[Macil]

Splitting this issue report out from #14 (comment).

$ npm install browserify browserify-incremental
$ echo "require('./b.js')" > a.js
$ echo "console.log('hi')" > b.js
$ ./node_modules/.bin/browserifyinc a.js -o app.js
$ rm b.js
$ ./node_modules/.bin/browserifyinc a.js -o app.js
$ node app.js
hi

[dcousens]

@jsdf is this an issue still?