Setting doctests and extests running automatically in Travis-CI

An edit of the Travis-CI build control file to cause the Iris doctests and extests to also automatically run when a new PR is submitted to SciTools/Iris. Overall this is a trivial change; simply a case of adding a directory change and three make commands (make clean html, make doctest, make extest) to the `script` section of `.travis.yml.`

There are, however, two caveats:
* For the doctests to pass, two items of documentation had to be updated to reflect a change to one of the sample data files. Further, the Iris sample data ref in `.travis.yml` has been pulled up to the most recent.
* The `make html` command is consistently erroring when run on Travis, causing builds to fail even when all build and test elements have passed. As a fix to this specific problem is outside of the scope of this PR, a workaround is employed in `.travis.yml`.