Docs / Update github-pages gem

[inancdokurel]

This updates the github-pages gem version in the docs gemfile.

This must be made to enable the kramdown version upgrade. Currently the github-pages gem is at version 210 which has a strict dependency to kramdown 2.3.0. The current version of github-pages increases the required package version.

This PR should be merged after the docs page branch migration.

This also resolves #275