[test] Use environment variables instead constants

.gitattributes

@@ -6,4 +6,3 @@
 .travis.yml export-ignore
 .php_cs export-ignore
 phpunit.xml.dist export-ignore
-phpunit.xml.travis export-ignore

.travis.yml

@@ -27,8 +27,8 @@ install:
   - travis_retry composer install --no-interaction --ignore-platform-reqs
 
 script:
-  - if [[ $EXECUTE_TEST_COVERALLS == 'true' ]]; then ./vendor/bin/phpunit -c phpunit.xml.travis --coverage-clover clover.xml ; fi
-  - if [[ $EXECUTE_TEST_COVERALLS != 'true' ]]; then ./vendor/bin/phpunit -c phpunit.xml.travis ; fi
+  - if [[ $EXECUTE_TEST_COVERALLS == 'true' ]]; then ./vendor/bin/phpunit --coverage-clover clover.xml ; fi
+  - if [[ $EXECUTE_TEST_COVERALLS != 'true' ]]; then ./vendor/bin/phpunit ; fi
   - if [[ $EXECUTE_CS_CHECK == 'true' ]]; then ./vendor/bin/php-cs-fixer fix -v --diff --dry-run ; fi
 
 after_script:

phpunit.xml.dist

@@ -28,7 +28,7 @@
              functionality works as expected. Such tests include those for
              Zend\Soap and Zend\Session, which require that headers not be sent
              in order to work. -->
-        <const name="TESTS_ZEND_OB_ENABLED" value="false" />
+        <env name="TESTS_ZEND_OB_ENABLED" value="false" />
 
 
     </php>