Enhance Travis CI setting

[peter279k]

According to this official blog post, the default dist on Travis CI build environment has changed into xenial.

And it will be failed on php-5.x versions during Travis CI build.

To resolve this issue, some suggestions I recommend are as follows:

• Dropping these php-5.x versions and let this package require stable php-7.1 version at least.
  And it will avoid this issue directly.
• Define the matrix setting to define the dist: trusty for these php-5.x versions during Travis CI build.

[Swop]

@peter279k Sorry for the (long) delay.

I've pushed a v3.0 today (which drop support for older php versions < 7.2), and switched to GitHub Actions instead of Travis.
Thanks for the notice.

[peter279k]

Thanks for your reply.

I will look at other improvements :).