Fix CI error in job for prefer-lowest packages

[o0h]

To fix cron build failure like https://travis-ci.org/github/Connehito/cake-sentry/jobs/708657380

What happened

As a result of the last build, each package remains in the cache with a non-prefer-lowest mode of dependency resolution.
This caused a situation where the version of PHPUnit conflicts with the version of PHPUnit when doing a downgrade of CakePHP.
The failure of the Composer's command made CI build an error.

What I do

To do prefer-lowest dependency resolution first, so that CakePHP downgrade can be done

[codecov-commenter]

Codecov Report

Merging #50 into 2.x will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                2.x      #50   +/-   ##
=========================================
  Coverage     89.18%   89.18%           
  Complexity       24       24           
=========================================
  Files             5        5           
  Lines            74       74           
=========================================
  Hits             66       66           
  Misses            8        8           

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