[travis] skip-lock during installation

[atb00ker]

Please correct me if i am wrong, but changing lock file during travis runs is not useful, right?

[coveralls]

Coverage remained the same at 99.588% when pulling 04d8c12cd6223f88c55a9cedb0e5878ddef4f346 on travis-qa into 75077b0 on master.

[okraits]

Yes, I think so, too. From my understanding, if we want to verify that our code works with the latest versions of the dependencies, then we should not use the lock file here but what is defined in the Pipfile. Thus your changes look correct to me.

[nemesifier]

Great, thanks 👍