Update govuk_template to 0.4.0

Removes the non-breaking space from the header.

Improves propositional navigation.

Crushes PNGs (with thanks to @jabley).

https://github.com/alphagov/govuk_template/compare/v0.3.7...v0.4.0
Merge pull request #145 from alphagov/update-govuk-template

Update govuk_template to 0.4.0