Use X-Vcap-Request-Id instead of X-Request-Id

* reverts post-PR change of X-Vcap-Request-Id => X-Request-Id

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>
Replace "magic strings" with constants

* eliminate duplicate constant declarations

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>
Bump godeps

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>