Keep browserify out of the repo

[romanenko]

Deployment breaks with error:

BrowserifyRails::BrowserifyError: browserify could not be found at /var/www/sample-project/releases/20140412164424/./node_modules/.bin/browserify. Please run npm install.

As I understand, currently, path to browserify executable is set as a constant. Maybe, we could make it configurable somehow? Or, maybe we should search for browserify executable like node does, or try to locate it on the server with which shell command for example?