Update parallel_tests to version 5.6.0

Author: depfu[bot]

Gemfile.lock

@@ -349,7 +349,7 @@ GEM
       omniauth (~> 2.0)
     ostruct (0.6.3)
     parallel (1.27.0)
-    parallel_tests (5.5.0)
+    parallel_tests (5.6.0)
       parallel
     parser (3.3.10.1)
       ast (~> 2.4.1)