Fixed dead links in doc section environment-variables.

Due to a missing `/` the links pointed to i.e. `docs/other/docs/other/core-configuration.html` producing a 404 / Page not found.
Move assets to assets/
Upgrade to middleman-hashicorp
Tell Rack to cache /assets
Merge pull request #1564 from mitchellh/sethvargo/middleman-hashicorp

Use middleman-hashicorp
Merge pull request #1537 from nifr/patch-1

Fixed dead links in doc section environment-variables.
Merge remote-tracking branch 'upstream/master'