Relax em-http-request and EM

Also locks down nats as the latest version at the time (0.4.28) locks
down EM to 0.12.10 which we want to bump.
Bump vcap_common and em-http-request

This commit also fixes create_http_request and cc_http_request along the
way, no longer assuming http.error is a string.
require nats helper when necessary

  We depended on the pollution before...