Mark Safari tests affected by intermittent failures as "allowed to fail"

[craigfowler]

Caused by #27 - tests using Safari which are affected by this issue need to be marked as permitted to fail in Travis' build matrix.

This is a short-term solution until the main problem can be resolved.

[craigfowler]

Actually, I'm going to close this as WONTFIX. I think it's dangerous allowing tests to fail indiscriminately. Whilst it's a bit more work, I should leave them failing so that it forces me to read the results and check nothing has broken.