Pin Pecan to 0.6.0 to fix binary secret decrypts

With the latest vesion of Pecan (0.7.0) attempts to GET binary secrets
with an Accept header of application/octet-stream result in a
Pecan/WebOb error trying to utf-8 decode the binary response.
See this link for more details:
https://bugs.launchpad.net/pecan/+bug/1373003

Change-Id: Id9469cb836cfc34636859d4cc8c0bce234e64e92
Related-Bug: #1373003
Merge "Pin Pecan to 0.6.0 to fix binary secret decrypts"