removing xdebug extension
merged branch pborreli/no-xdebug-travis (PR #7398)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #7398).

Commits
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df99ecf [travis-ci] removing xdebug extension

Discussion
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[travis-ci] removing xdebug extension

we don't need it (no coverage), and removing it speeds travis build by 1 minute approx (composer + icu + phpunit)