Upgrade PHP version on Travis

[kytrinyx]

The build on Travis is failing because the phptest.phar that gets downloaded is incompatible with the version of PHP that we've specified in the .travis.yml file.

PHPUnit 7.0.0 by Sebastian Bergmann and contributors.
This version of PHPUnit is supported on PHP 7.1 and PHP 7.2.
You are using PHP 7.0.25 (/home/travis/.phpenv/versions/7.0.25/bin/php).
make[1]: *** [test-assignment] Error 1

[kytrinyx]

This passes on Travis, but runs with warnings in meetup and series:

Warning: The use statement with non-compound name 'DateTime' has no effect in /tmp/meetup.6BZBp1kvsu/meetup_test.php on line 5

Warning: The use statement with non-compound name 'DateTime' has no effect in /tmp/meetup.6BZBp1kvsu/meetup.php on line 3

Warning: The use statement with non-compound name 'Exception' has no effect in /tmp/series.NmEAIpwb5W/series_test.php on line 3

Warning: The use statement with non-compound name 'Exception' has no effect in /tmp/series.NmEAIpwb5W/series.php on line 3

[kytrinyx]

Ok, I've updated the exercises to remove the warnings on make test.

[kytrinyx]

While I'm at it I'm going to see if we can upgrade to 7.2.

[kytrinyx]

Ok, that didn't work, as calling assert() with strings is deprecated. I'm backing out the 7.2 change, but will open an issue about it.

This should be good to go.