Update robotframework-maven-plugin to 1.6.0

[php-coder]

See:

• https://github.com/robotframework/MavenPlugin/releases/tag/1.5.2
• https://github.com/robotframework/MavenPlugin/releases/tag/1.6.0

[php-coder]

We can activate dotted output mode with 1.6.0: robotframework/MavenPlugin#67