bump pecan minimum version number to 0.8.0

Several bugs have been fixed in more recent releases of pecan[1].  The
JSON handling in 0.8.0 will make it easier to transition to pecan.

[1] http://pecan.readthedocs.org/en/latest/changes.html

Change-Id: I773b52e2a0dcbf7e17dc9653b069ff0ee989e040
Merge "bump pecan minimum version number to 0.8.0"