bumping Plek to 1.0.0

In doing this I detected some tests that are assuming GOVUK_ASSET_HOST
is not set (or set empty). We're in the process of trying to establish a
standard way of setting up environment variables for each app (currently
govuk_setenv is available) therefore I have modified the tests to
function correctly when this variable is set. When not set,
NoConfiguration exception will be thrown - same goes for
GOVUK_APP_DOMAIN.

@heathd worked with me on some of this
Updating README for breaking change
forgot to push lock file updates for slimmer
whitehall is now the app name
Fix bad merge
Add required environment variables to Jenkins
Formatting of Jenkins scripts
Update README to reflect foreman/bowler config
use GOVUK_ASSET_ROOT, not GOVUK_ASSET_HOST

GOVUK_ASSET_HOST was the original name for this variable. However, it refers to a URL, not a hostname.
We therefore want to move to the variable GOVUK_ASSET_ROOT, which stores the same thing but is better named.
use correct name in jenkins scripts

...although I quite like GO.UK as a website name :)
Use govuk_setenv correctly for Jenkins

Note: this depends on the `whitehall` directory existing on its own in
the env.d directory, as it will break in its current state.

This work has been done in alphagov/puppet#217 .
Remove dependency on govuk_setenv

We're probably not going to use this on the CI boxes, so we should just
set the desired environment variables explicitly.
Merge pull request #128 from alphagov/bump-plek

Update Plek to 1.0