Test on PHP 7.1

[GrahamCampbell]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
License	MIT

What's in this PR?

Adds PHP 7.1 to the travis test matrix.

Why?

PHP 7.1 is coming later this year, and it would be good to verify compatibility.

Checklist

• [ ] Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• [ ] Documentation pull request created (if not simply a bugfix)

  To Do

• [ ] If the PR is not complete but you want to discuss the approach, list what remains to be done here

[sagikazarmark]

Same fix is necessary here IMO.

[sagikazarmark]

Thanks