Update ComposerTask code & documentation

[jawira]

This PR makes command attribute from ComposerTask required.

Could you double check this PR please?

Please note that indentation in xml files is two spaces now, this is required by .editorconfig.

As always I'm available for any comments or remarks.

Solves #1163

[codecov]

Codecov Report

Merging #1175 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@             Coverage Diff              @@
##             master    #1175      +/-   ##
============================================
+ Coverage     47.09%   47.09%   +<.01%     
- Complexity     9972     9973       +1     
============================================
  Files           505      505              
  Lines         24480    24483       +3     
============================================
+ Hits          11529    11531       +2     
- Misses        12951    12952       +1

Impacted Files	Coverage Δ	Complexity Δ
classes/phing/tasks/ext/ComposerTask.php	79.54% <75%> (-0.95%)	15 <0> (+1)

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