Add minimum-stability & prefer-stable to get chullo:dev

[whikloj]

GitHub Issue: N/A

Still trying to get correct chullo into Crayfish related to #74 test failures

What does this Pull Request do?

1. Update all lockfiles to PHP 7.2
2. Update chullo/crayfish-commons to dev branches
3. Update travis testing to remove 7.1 and add 7.3 as an allowed failure.

What's new?

How should this be tested?

Travis really.

Interested parties

@Islandora-CLAW/committers

[codecov]

Codecov Report

Merging #75 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                dev      #75   +/-   ##
=========================================
  Coverage     94.59%   94.59%           
  Complexity      159      159           
=========================================
  Files             9        9           
  Lines           647      647           
=========================================
  Hits            612      612           
  Misses           35       35

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[whikloj]

If this looks good, I'll make one to Crayfish-Commons and Chullo to update for 7.2 and test against 7.3

[dannylamb]

Let's do this