Enhancement: Let dependabot update GitHub actions

[localheinz]

This PR

• lets dependabot update GitHub actions

💁‍♂ For reference, see

• https://github.com/localheinz/php-library-template/pull/132#discussion_r343548273
• https://github.com/dependabot/dependabot-core/tree/master/github_actions

[codecov]

Codecov Report

Merging #136 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             master   #136   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity        3      3           
=======================================
  Files             1      1           
  Lines             6      6           
=======================================
  Hits              6      6

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