Fix broken tests by upgrading Slimmer

Older versions of slimmer used a different host for assets
during tests. That host now has an SSL error and was breaking
test.
Merge pull request #106 from alphagov/fix-js-test-failures

Fix broken tests by upgrading Slimmer
Child benefit calculator: change boundary for percent tax change
Merge pull request #105 from alphagov/change-boundary-tax-calculation

Change boundary for percent tax change
Add a startup script and update README

Same as lots of other repos, means you can start the app on the
right port without having to know what that is.

Add usage of the startup script to the README, how to view content
and how to run tests while i'm at it.