Update travis config for minimum version scenario.

phpunit 7.0.0 lost the ability to recursively create folders (fixed in 7.0.3), so we need to manually create the 'build' directory before running phpunit when running against minimum versions.

.travis.yml

@@ -20,7 +20,7 @@ matrix:
 
 install:
   - if [[ $MINIMUM_VERSIONS = false ]]; then composer install; fi
-  - if [[ $MINIMUM_VERSIONS = true ]]; then composer update --no-interaction --prefer-lowest; fi
+  - if [[ $MINIMUM_VERSIONS = true ]]; then composer update --no-interaction --prefer-lowest; mkdir build; fi
 
 script:
   - composer run test