Adding PHP 5.6 to travis-ci tests
minor #10248 Adding PHP 5.6 to travis-ci tests (pborreli)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10248).

Discussion
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Adding PHP 5.6 to travis-ci tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

PHP 5.6.0alpha1 is available on travis-ci
tests are green

Commits
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7083a30 Adding PHP 5.6 to travis-ci tests