Remove Perl directives from .travis.yml

Merged Marek Szuba requested to merge travis_no_perl into version/2.4

Created by: mkszuba

On the one hand perl-5.10 is not available under Xenial (which is in the process of becoming the default Travis build environment) so requesting it would cause a build failure, on the other we do not actually USE perl here - all the testing happens inside a Docker container. Just get rid of this.

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