Upgrade pitest-maven-plugin 1.15.3 -> 1.15.6 (#984)

See:
- https://github.com/hcoles/pitest/releases/tag/1.15.4
- https://github.com/hcoles/pitest/releases/tag/1.15.5
- https://github.com/hcoles/pitest/releases/tag/1.15.6
- hcoles/pitest@1.15.3...1.15.6

pom.xml

@@ -1395,7 +1395,7 @@
                 <plugin>
                     <groupId>org.pitest</groupId>
                     <artifactId>pitest-maven</artifactId>
-                    <version>1.15.3</version>
+                    <version>1.15.6</version>
                     <configuration>
                         <excludedClasses>
                             <!-- AutoValue generated classes. -->