Update dependencies and code style tools

[phil-davis]

1. require PHP 7.1 or later and phpunit 7
2. test on only PHP 7.1 through 7.4

Note 1: CI currently runs phpstan - good stuff and it passes at the current level 0.

Note 2: there is already a config for php-cs-fixer but it is not running in CI. There are lots of edits that it wants to make. I will do that in a separate PR #492 because maybe nobody wants to apply such a lot of changes.

[codecov]

Codecov Report

Merging #491 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #491     +/-   ##
===========================================
- Coverage     98.87%   98.68%   -0.2%     
+ Complexity     1811     1757     -54     
===========================================
  Files            66       66             
  Lines          5178     4271    -907     
===========================================
- Hits           5120     4215    -905     
+ Misses           58       56      -2

Impacted Files	Coverage Δ	Complexity Δ
lib/TimeZoneUtil.php	97.67% <0%> (-2.33%)	23% <0%> (ø)
lib/Parser/XML/Element/KeyValue.php	84.61% <0%> (-1.1%)	4% <0%> (ø)
lib/DateTimeParser.php	98.34% <0%> (-0.47%)	55% <0%> (ø)
lib/Component/VCalendar.php	94.59% <0%> (-0.35%)	64% <0%> (-13%)
lib/Property/Time.php	97.29% <0%> (-0.27%)	13% <0%> (-1%)
lib/Parser/Json.php	96.77% <0%> (-0.2%)	18% <0%> (-2%)
lib/BirthdayCalendarGenerator.php	98.07% <0%> (-0.17%)	18% <0%> (ø)
lib/Property/Text.php	99.12% <0%> (-0.16%)	40% <0%> (-3%)
lib/Property/FloatValue.php	96% <0%> (-0.16%)	9% <0%> (ø)
lib/FreeBusyGenerator.php	98.1% <0%> (-0.07%)	90% <0%> (-5%)
... and 37 more

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[staabm]

Lgtm