Circle CI badge should show status of master branch by default

[chris48s]

As noted in #1859 the standard behaviour for our Circle CI badge is to show the status of the latest build on any branch. However, for consistency with the behaviour of other build service badges and Circle CI's own badges the standard behaviour should be to show the status of the latest build on the repo's default branch.

We should also provide some way to maintain the current behaviour as some users may rely on this.