Depend on Github Pages gem

Replace the direct dependency on Jekyll with one for Github Pages, where
this site is hosted, with the aim of improving development-production
parity.

As of version 21 of the Github Pages gem, Jekyll 2.2.0 is supported. This
change upgrades us from Jekyll version 2.0.3 to 2.2.0.
Merge pull request #3 from gds-operations/use-github-pages-gem

Depend on Github Pages gem