Make php-dontenv work with older php versions

[robert197]

This extension used to work for php 5.3+.
It would be nice if we could make it work with other php versions than php 7.0+.
This way we could integrate php-dontenv in other frameworks. e.g. testing-frameworks.

[josegonzalez]

You can use release 2.1.0. The latest 3.x line simply adds support for apache_getenv and apache_setenv, which are not testable below 7.x, hence why I dropped support.

[josegonzalez]

I'm going to lower the version supported by not requiring tests for apache_* methods. I'll consider them untested in lower than 7.0, but all the logic should work fine.

[josegonzalez]

#38