Allow custom RAILS_ENV

[yskkin]

When executing RAILS_ENV=staging bin/rails assets:precompile, following error is raised. This PR fix it.

[..snip..]
Webpacker is installed 🎉 🍰
Using /var/www/myapp/releases/20180216111626/config/webpacker.yml file for setting up webpack paths
Compiling…
Compilation failed:

/var/www/myapp/releases/20180216111626/node_modules/@rails/webpacker/package/config.js:15
  delete defaultConfig.extensions
  ^

TypeError: Cannot convert undefined or null to object
    at Object.<anonymous> (/var/www/myapp/releases/20180216111626/node_modules/@rails/webpacker/package/config.js:15:3)
    at Module._compile (module.js:641:30)
    at Object.Module._extensions..js (module.js:652:10)
    at Module.load (module.js:560:32)
    at tryModuleLoad (module.js:503:12)
    at Function.Module._load (module.js:495:3)
    at Module.require (module.js:585:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/var/www/myapp/releases/20180216111626/node_modules/@rails/webpacker/package/rules/babel.js:2:24)
    at Module._compile (module.js:641:30)
rake stderr: Nothing written

[doits]

Not that easy, dupe of #1265 ;-)

[yskkin]

I should have searched PR more thoroughly 🙇