Don't explicitly export {RAILS,RACK}_ENV
Don't explicitly export {RAILS,RACK}_ENV

We don't need to explicity export `RAILS_ENV` and `RACK_ENV` in
`govuk_spinup` as these are already set by `govuk_setenv`:

    mattbostock@staging-frontend-1:~$ govuk_setenv manuals-frontend printenv RAILS_ENV
    production
    mattbostock@staging-frontend-1:~$ govuk_setenv static printenv RAILS_ENV
    production
    mattbostock@staging-frontend-1:~$

I've verified this by removing these lines from `govuk_spinup` then
running:

    sudo cat /proc/$(pgrep frontend)/environ

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet