Removed postinstall-build in favor of npm prepare script

[guilhermehn]

Summary

Proposed change:

postinstall-build is now deprecated because of npm@4 prepare lifecycle script.

This will break for people using nunjucks as a git dependency with npm < 4. As node 6 end-of-life is coming close, less people will be using npm@3.x. This will help with the third Purpose item:

• Works in all node releases that are
  actively maintained by the Node Foundation

Also, npm@5 is the actual LTS version, so npm@3 and npm@4 should be considered deprecated.

Closes #1167.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[edevil]

Any feedback on this?

[fdintino]

This looks good to me. I'd like to check one additional thing, but then it should be ready to go.