fix(travis): Install missing mysql

[klausi]

Try to get Travis CI running again.

[klausi]

Hopefully this fixes the test fail. I found a security issue where we use $accessResult->isForbidden() which does not do what you think it does. We always need to use $accessResult->isAllowed() to check for access.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (8.x-4.x@0bf46bc). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             8.x-4.x     #920   +/-   ##
==========================================
  Coverage           ?   66.43%           
  Complexity         ?      587           
==========================================
  Files              ?       92           
  Lines              ?     1299           
  Branches           ?        0           
==========================================
  Hits               ?      863           
  Misses             ?      436           
  Partials           ?        0

Impacted Files	Coverage Δ	Complexity Δ
...GraphQL/DataProducer/Entity/EntityLoadMultiple.php	66.66% <100%> (ø)	23 <0> (?)

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