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.
fix: Respect metadata supplied for GCE builders.

Previously, any per instance metadata supplied via the GCE builder
was ignored.

Test plan:

- make test
- Manual testing via:
-- Created a packer config that contained a GCE builder with custom
   metadata set.
-- Ran `packer build`.
-- Verified the instance had the correct metadata in the GCE console.
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 pull request #1563 from mpage/fix-gce-metadata

fix: Respect metadata supplied for GCE builders.
fix a minor typo in platforms.html
Merge pull request #1573 from rozha/master

fix a minor typo in platforms.html