Travis tool sometimes checks wrong commits

[TomSeldon]

For a pull request build (e.g. when merging develop into master) we use the environment variable TRAVIS_COMMIT_RANGE to work out what commits we need to check.

This seems to work correctly in most cases, however it's been observed that in some cases that SHA range (or perhaps and more likely the way we're interpreting it) will include commits that aren't part of the pull request.

Difficult to replicate and I'm not aware of any specific replication steps, but it's been seen often enough to be an issue.