Travis [release/97]: use release/97 branch of dependent Ensembl repos

Created by: mkszuba

Description

Make Travis use the release/97 branch of dependent Ensembl repositories rather than master.

Use case

At present Travis builds of ensembl-rest always pull respective master branches even when ensembl-rest itself is on a release branch. This will no longer be the case starting with release 98, and it does make sense to backport the change to release 97 in light of the ongoing issue with failing Variation tests, which will have to be fixed for both master and release/97 and which may or may not require changes in ensembl-variation in addition to / rather than ensembl-rest.

Benefits

Travis builds of ensembl-rest for release 97 will no longer pull in master branches of Ensembl repositories, which have already branched to 98 and will diverge more and more from the 97 code base.

Possible Drawbacks

None I can think of.

Testing

Have you added/modified unit tests to test the changes?

No.

If so, do the tests pass/fail?

N/A

Have you run the entire test suite and no regression was detected?

Change affects Travis only. The build on the pushed branch has successfully cloned the dependent repositories and failes in exactly the same way as before the change (i.e. on variation.t failures).

Changelog

Are you changing the functionality of an endpoint? If so, please give a one line summary for the public facing changelog.

Changes only affect the execution of tests on Travis, no endpoint-related changes of any sort have been involved.