Travis: explicitly use Ubuntu Trusty for builds

Matthieu Muffato's experiments with ensembl-hive have shown that in
spite of https://docs.travis-ci.com/user/languages/perl/ claiming
Travis supporting perl-5.8 and newer, the oldest version supported under
the Xenial environment is 5.22. This has since been confirmed by a post
on Travis community forums:
https://travis-ci.community/t/failure-with-perl-5-16-5-18-5-20/2458 .

We are presently not ready to drop support for Perl versions older than
what Travis makes available under Xenial, therefore let us for the time
being make sure we continue to use Trusty for now.

Release-98 version bump

For some reason ensembl-io test ontology database went out of sync some
time prior to release 97. See the discussion of PR #85.

Source: release/98 branch of ols-ensembl-loader and master branches
of all other relevant Ensembl repositories, pulled a few minutes ago.
Compara, Variation and ontology schemas all have already been bumped to
version 98.

README: Use the same badge syntax as for 'ensembl'

i.e. explicitly specify the branch

ancestral_allele update to handle Variation API changes.

[ENSCORESW-3077] Travis: Unify configuration of Slack notifications

ensembl Utils::IO into ensembl-io

Looks good to me, the errors are related to th changes to the schema in the ensembl repo which is fixed in a separate PR.

Check response status code instead of message for file fetch requests