Update .appveyor.yml

[ruudboon]

Hello!

• Type: new feature
• Link to issue: [NFR] PHP74 support for windows #84

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist.
• I wrote some tests for this PR.

Small description of change: Added PHP 7.4 to app veyor

Thanks

[codecov]

Codecov Report

Merging #85 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development      #85   +/-   ##
============================================
  Coverage        80.52%   80.52%           
============================================
  Files                5        5           
  Lines             6538     6538           
============================================
  Hits              5265     5265           
  Misses            1273     1273

Flag	Coverage Δ
#unittests	80.52% <ø> (ø)	⬆️

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[ruudboon]

@sergeyklay When this is merged, can we somehow trigger a 7.4 windows build on the v1.3.2 tag so we can add it to the release?

[sergeyklay]

@ruudboon We'll need a new release. Let me do it

[ruudboon]

@sergeyklay Ok. Thnx