Ensure test suite is run before releasing GOV.UK Frontend

[NickColley]

At the moment it is possible to have a failing master branch.

1. Contributor 1 pushes PR 1
2. PR 1 passes CI
3. Contributor 2 pushes PR 2 with breaking changes that 1 relied on
4. PR 2 passes CI and is merged into master
5. PR 1 will not now be failing since Travis only runs on updates to an individual PR, not master changes.

Considered solution: Force branches to be up-to-date with master

We ruled this out since it would mean with so many PRs happening we'd have to rebase very often which would slow down the team.

Another option is to ensure that before releases our test suite is ran.

We need to update the build scripts or documentation to include this step.

[36degrees]

We ruled this out since it would mean with so many PRs happening we'd have to rebase very often which would slow down the team.

It would also provide an extra barrier to contribution in the future, as requiring contributors to continually rebase their PRs adds extra work to what should be a relatively simple process.