Upgrade dependencies

We’d like to be able to run this on heroku, so that the team
responsible for the governance project can publish their changes and do
user research before putting their content live.

There are various issues with our current dependency versions which are
preventing that.

Upgrading to latest versions of things and then we’ll make pull requests
if bugs / features are still an issue.
Fix deprecation warning

```
Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.
```
Merge pull request #417 from alphagov/feature/gem-upgrades

Upgrade dependencies