drop PHP 7.1 + Python 2.7 from TravisCI builds

[jmckenna]

• PHP 7.1 was end-of-life in 2018, and ended security fixes at end of 2019
• Python 2.7 ended security releases at end of 2019

Remaining tests are:

• PHP 7.4, 7.3, 7.2
• Python 3.8, 3.7, 3.6

[geographika]

@jmckenna - is this to speed up the Travis builds or part of dropping Python 2.7 support?
I think we can support Python 2.7 as long as SWIG does.
Note Python 2.7 will still be tested on Appveyor, so maybe that is enough.

[jmckenna]

@geographika my goal here was to remove long end-of-life versions from the list of tests on TravisCI. A nice benefit is that it cleans up the total number of tests, and takes a little shorter to complete as you mentioned. My goal was not to 'officially' remove support for any versions.