Adding X-Travis-Repo-Slug header to webhook requests

so that consuming the request body isn't necessary to calculate the
validity of the Authorization signature.
Switching to a non-X header per rfc6648

plus removing rand in the spec per feedback from @henrikhodne.
Merge pull request #267 from modcloth/include-x-travis-repo-slug-header-for-webhooks

Adding Travis-Repo-Slug header to webhook requests