Extended the env variables for Iris's travis-ci testing.

Splits the tests up further in travis-ci. Adds the system tests to the mix.

Example before: https://travis-ci.org/SciTools/iris/builds/38275701
Example after: https://travis-ci.org/pelson/iris/builds/38798350

Max run-time drops from 11m50s to 9m10. A finger in the air measurement suggests that the total install time is around 3 mins currently.
For the same reason we run the default tests on the installed version, wouldn't it be better to run the system tests on the installed version?