Fix issues #126 produced by the inclusion of phpstan/extension-installer:
Changes the version of php for travis checks since phpStan extension-installer requires ^php 7.1 in order to work, and as we are setting the composer to php 7.3, I added that version too.
Fix issues #126 produced by the inclusion of phpstan/extension-installer: Changes the version of php for travis checks since phpStan extension-installer requires ^php 7.1 in order to work, and as we are setting the composer to php 7.3, I added that version too.