Installing av package doesn't work on OS X on Travis

[ageorgou]

Low priority.

Recently, the OS X runs on Travis have started failing because the av module is not found (this only affects one test -- all the others pass). See example failing job.

This is strange, because we do install av with conda install -n test-environment av tifffile -c conda-forge, and nothing in the log indicates that this fails. However, it is not available to import when the tests run. The installation and import work fine on Linux jobs.

Interestingly, the package installed on OS X is now version 7.0.1, whereas for the Linux jobs it is 8.0.2. Even more strangely, it used to be 8.0.2 on OS X until around a month ago (example job) and those jobs also succeeded. The first failing job had the downgraded version, so maybe there's something wrong with that version.