update heroku deployment guide for webpack

[happysalada]

What

update the heroku deployment guide for webpack

Comments

• by default the buildpack doesn't use node's latest LTS so it's running node 6.9, I thought it would be a nice improvement to use the LTS version (even if that version changes)
• It seems phoenix wants to use npm by default so I kept the compile file with npm.

refs #2827

[Gazler]

I like the idea, but I don't think we should specify exact versions in the guide - we will have to update every time a new node version is released. Instead, we should simply link to the configuration section of the buildpack with a note.

Something like:

This phoenix static buildpack pack can be configured to change the node version and compile options. Please refer to this configuration section for details.

[happysalada]

@Gazler updated the guide accordingly.
I didn't squash my commits to make the PR more readable. Let me know if you need me to.

[happysalada]

@Gazler just rebased off latest master. Let me know if you want any other changes.

[Gazler]

Thanks <3