Add GitLab CI

• Different set of boxes, there isn't a 7.2 in the current series and doesn't seem like that'll changes
• Correct PHPUnit version for PHP version, as Travis handles

Fixes #2 (closed)

bin/install-wp-tests.sh

@@ -95,7 +95,7 @@ install_wp() {
 install_test_suite() {
 	# portable in-place argument for both GNU sed and Mac OSX sed
 	if [[ $(uname -s) == 'Darwin' ]]; then
-		local ioption='-i .bak'
+		local ioption='-i.bak'
 	else
 		local ioption='-i'
 	fi