Travis php71

[remicollet]

Discovered in Fedora QA,
https://apps.fedoraproject.org/koschei/package/php-lessphp?collection=f26

Since PHP 7.1, test suite fails with "A non-numeric value encountered", which allow to discover this bug.

The PR also enable PHP 7.0 and 7.1, and allow latest PHPUnit version

$ php70 vendor/bin/phpunit  --verbose
PHPUnit 5.6.5 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.0.14RC1
Configuration: /work/GIT/lessphp/phpunit.xml

............................................                      44 / 44 (100%)

Time: 85 ms, Memory: 8.00MB

OK (44 tests, 54 assertions)

$ php71 vendor/bin/phpunit  --verbose
PHPUnit 5.6.5 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.1.0RC6
Configuration: /work/GIT/lessphp/phpunit.xml

............................................                      44 / 44 (100%)

Time: 103 ms, Memory: 8.00MB

OK (44 tests, 54 assertions)

/cc @siwinski