Add support for PHP 7.4

[mmachatschek]

Purpose

Change travis configuration and add support for PHP 7.4

Open Questions and Pre-Merge TODOs

• [x] composer lint and composer fix was executed.
• [x] Tests were written and pass with 100% coverage.
• [ ] A issue with a detailed explanation of the problem/enhancement was created and linked.

[codecov-io]

Codecov Report

Merging #113 into develop will not change coverage. The diff coverage is 100%.

@@           Coverage Diff            @@
##             develop   #113   +/-   ##
========================================
  Coverage        100%   100%           
  Complexity       267    267           
========================================
  Files             49     49           
  Lines            701    700    -1     
========================================
- Hits             701    700    -1

Impacted Files	Coverage Δ	Complexity Δ
...rt/Exceptions/EmptyElementsNotAllowedException.php	100% <100%> (ø)	1 <0> (ø)	:arrow_down:

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[mmachatschek]

@howard What do you think about dropping support for 1.x branch?

[howard]

@mmachatschek 1.x is still in active use for some shop system plugins that have a very old PHP version baseline, so it's not time to do that yet.

Please limit the changes to what's necessary to make libflexport (2.x) compatible with PHP 7.4, without traces of introducing a new major version. That's a new topic that will soon get its own milestone issue to collect ideas and outline the plan for libflexport 3.0.

[howard]

@mmachatschek Your input for libflexport 3.0 is welcome in #114.

[mmachatschek]

@howard I updated the code again. I added support for PHP 7.1, as we would need to release a major version if we drop PHP 7.1? If not, please tell me. I would gladly update the code again.