Update ruby-rest-client to 1.8.0, security fix.

# 1.8.0

- Security: implement standards compliant cookie handling by adding a
  dependency on http-cookie. This breaks compatibility, but was necessary to
  address a session fixation / cookie disclosure vulnerability.
  (#369 / CVE-2015-1820)

  Previously, any Set-Cookie headers found in an HTTP 30x response would be
  sent to the redirection target, regardless of domain. Responses now expose a
  cookie jar and respect standards compliant domain / path flags in Set-Cookie
  headers.


(taca)