Optionally install GOV.UK apps used by perf-plat

Run install.sh with GOVUK_DEPS=1 to also clone
repos from GOV.UK which the performance platform
is using eg. static and signonotron2
Remove signonotron2 from GH list
Merge pull request #1 from alphagov/Add_govuk_repos_to_install_script

Add govuk repos to install script