Add big-endian case (arch: s390x) as allow_failures.

[junaruga]

It is related to #355 .

#355 (comment)

It won't break the amd64 test cases, but it will break the build as a whole. Once the remaining issues have been tracked down and fixed it will be useful to turn this on so that they don't regress, but it would be rather distracting at the moment.

This PR is to add s390x case as allow_failures with fast_finish: true option.

allow_failures is to ignore the result of the test case that is set as "allow_failures".
As a result, as Travis is green (succeeded) even when the s390x case is failed, "it will not break the build as a whole".

fast_finish: true option is return the entire result without waiting the test case that is set as "allow_failures". As a result, we are not disturbed by the case.

Here is my forked repository's result.
https://travis-ci.org/junaruga/htslib/builds/611619509

I admit that the s390x case is slow at the moment.
But we can see the useful result.

https://travis-ci.org/junaruga/htslib/jobs/611619519#L335

Unexpected failures: 4

[junaruga]

I would close this PR as I got feedback from John.
I like you adapt it when you like it in the future.
Sorry for disturbing it.