travis: test all supported versions

[grooverdan]

Here we use docker to test the upstream supported versions and the MySQLTuner oldest version (5.5).

Turns out the #531 execute permissions are useful for the test.

The travis build looks like https://travis-ci.com/github/grooverdan/MySQLTuner-perl/builds/215009119

Migrating from travis-ci.org to travis-ci.com free account runs all the tests without the delay of several hours to get the results. Might even be able to enable them on pull requests too. The notice on travis-ci.org says migration in a couple of weeks anyway.

To migrate:
https://travis-ci.org/account/repositories as the account holder and top right "Sign up for beta". Adjust travis links in README.md afterwards.

closes #531, #532, #533

[jmrenouard]

Hi @grooverdan

There are conflict in this commit.

Can you help us to undestand your changes based on last versions ?

Thanks by advance

[grooverdan]

Done.

Being docker I did hit this https://www.docker.com/increase-rate-limits?utm_source=docker&utm_medium=web%20referral&utm_campaign=increase%20rate%20limit&utm_budget= once. However I'm hoping that was just because of my testing and not tied to the travis's infrastructure having that limit. Small disclaimer. I think its pretty safe but we can revert if we see too many test failures on rate limit.

Recommend "signing up for beta" https://travis-ci.com/github/major/MySQLTuner-perl

https://travis-ci.com/github/grooverdan/MySQLTuner-perl complete instantly however https://travis-ci.org/github/major/MySQLTuner-perl/builds/756637615 is still waiting.