Update Wyam to 2.2.7
Update Wyam to 2.2.7

Bumps Wyam to the most recent version which should address the intermittent package installation failures we've started to see.

Might need to kick the PR build again in a few minutes if the NuGet gallery hasn't indexed the new Wyam version yet.
cake-build/website,cake-build/website,cake-build/website