Relax assumptions in webpack:compile rake tasks

[pariser]

• Don't assume that ::Rails.configuration.webpack.binary specifies a webpack path relative to Rails root.
• Don't assume that we know the exact webpack binary location (allow webpack executable to be on $PATH, in which case we no longer die with a File.exist? error).

[mipearson]

I like this, but it needs to support the current configuration by default so that it doesn't break for existing users.

In our configuration we don't install webpack (or webpack dev server) globally.

[juanca]

If the compile script leveraged an npm task then I think this can completely be avoided? Perhaps there should be a npm run webpack:compile task which compiles with a production flag(s).

[mipearson]

I'm going to be bringing this in to an API-breaking 1.0-pre branch shortly.

The delay on this (and other) pull requests is that I've been cautious to merge anything that could break functionality for existing users.

[pariser]

Cool! Let me know if and how I can help with this.