Php 7.4 support

[AydinHassan]

• Upgrade PHPUnit and fix some deprecations. PHPUnit 7 does not support PHP 7.4. I've had to expose a few properties because of deprecations regarding methods for accessing private properties in PHPUnit. Probably for the best.
• Drop 7.1 testing from travis and add 7.4. We can still support 7.1 but we can't test on travis both 7.1 and 7.4 with the same version of PHPunit.

[codecov-io]

Codecov Report

Merging #183 into master will increase coverage by 0.04%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #183      +/-   ##
============================================
+ Coverage     93.25%   93.29%   +0.04%     
- Complexity      453      460       +7     
============================================
  Files            26       26              
  Lines          1393     1402       +9     
============================================
+ Hits           1299     1308       +9     
  Misses           94       94

Impacted Files	Coverage Δ	Complexity Δ
src/MenuItem/LineBreakItem.php	100% <100%> (ø)	11 <1> (+1)	:arrow_up:
src/MenuItem/SplitItem.php	100% <100%> (ø)	43 <1> (+1)	:arrow_up:
src/CliMenu.php	93.04% <100%> (+0.1%)	105 <3> (+3)	:arrow_up:
src/MenuItem/AsciiArtItem.php	100% <100%> (ø)	18 <2> (+2)	:arrow_up:
src/Input/InputIO.php	96.87% <0%> (-0.08%)	25% <0%> (ø)

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