Update govuk_admin_template to 1.1.3

This fixes an issue where the GOV.UK brand would not display using the
correct font in Internet Explorer or Chrome on Windows. For other
changes check the changelog in `govuk_admin_template`.

Sass is also upgraded from 3.2 to 3.4 in this release necessitating
adding an explicit import of '_shims' before the mixin %contains-floats
can be used.

Sass's changelog
(http://sass-lang.com/documentation/file.SASS_CHANGELOG.html#backwards_incompatibilities__must_read)
doesn't specifically mention anything I can see that would cause this,
but it's two breaking changes difference.
Merge pull request #272 from alphagov/update-govuk-admin-template

Update govuk_admin_template to 1.1.3