Do not use tables prefix when running tests on travis.

tests/PHPUnit/config.ini.travis.php

@@ -17,7 +17,8 @@ username = root
 password =
 dbname = piwik_tests
 adapter = PDO\MYSQL
-tables_prefix = piwiktests_
+; no table prefix for tests on travis
+tables_prefix = 
 
 [log]
 log_writers[] = file