Updated command to upload coverage in Travis CI config

[michalbundyra]

• Occasionally upload fails, so using travis_retry gives us a couple retries.

• Occasionally, the executable bit is not set on vendor binaries; this ensures the command will still run.

• per @weierophinney

https://github.com/zendframework/zend-diactoros/pull/288#discussion_r161881452

[weierophinney]

Thanks, @webimpress