UPDATE requirements to not include @dev

[muskie9]

• Update phpunit/PHPUnit to phpunit/phpunit
• Remove reference to silverstripe/recipe-cms as this should be handled by dnadesign/silverstripe-elemental
• Remove reference to dnadesign/silverstripe-elemental as this should be handled by elements pulled in by this recipe
• Remove reference to silverstripe/recipe-plugin as it is infirred

[codecov[bot]]

Codecov Report

Merging #111 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #111   +/-   ##
=========================================
  Coverage     84.61%   84.61%           
  Complexity        2        2           
=========================================
  Files             2        2           
  Lines            13       13           
=========================================
  Hits             11       11           
  Misses            2        2

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