Fix travis configuration + DateTime-bug

[tjveldhuizen]

After the merge of #36, I saw @steverhoades struggling with the Travis setup. While I'm not experienced with Travis, I decided to dive into it, because I really would like to upgrade my environment to PHP 8.

There where three causes for the failing runs:

• The matrix was effectively defined twice, due to the extra environment variable now the tests ran also for incomplete env-setups
• Tests where failing because of deprecations in older environments, for which a decission should be made if the will be maintained (and the deprecations should be fixed) or not (deprecations are fine). That's why I have changed the Travis-config to ignore deprecations
• After fixing those issues, I ran into an error in the specific combination of oAuth2-server 8.0 and lcobucci/JWT 3.4.*, which I've fixed

And after some failed runs:

• Updated Ubuntu-version to Xenial, to support PHP 5.6 as well as 8.0
• Allowed to use version 9.5 of PHPUnit, since 5.0 does not support PHP 8.0

[steverhoades]

Thanks for taking a look at this, I have been traveling and wasn't able to finish that integration. I'll take a look at this ASAP.

[steverhoades]

LGTM, Thanks for this!

[tjveldhuizen]

@steverhoades Is it possible to do a release today? It would fit perfectly in our update schedule this week ;)

[steverhoades]

@tjveldhuizen released as 2.0.0