Drop PHP 7.1

[phil-davis]

1. Bump bamarni/composer-bin-plugin to 1.4
2. Drop PHP 7.1 from CI (including up-to-date drone starlark code)

[codecov]

Codecov Report

Merging #148 into master will decrease coverage by 0.38%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #148      +/-   ##
============================================
- Coverage     99.22%   98.83%   -0.39%     
  Complexity       60       60              
============================================
  Files             8        8              
  Lines           258      258              
============================================
- Hits            256      255       -1     
- Misses            2        3       +1     

Impacted Files	Coverage Δ	Complexity Δ
lib/Notification/Notifier.php	95.45% <0.00%> (-4.55%)	6.00% <0.00%> (ø%)

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