Use fixx mkdocs version (#240)

.travis.yml

@@ -1,7 +1,7 @@
 language: groovy
 sudo: false
 install:
-- pip install --user mkdocs mkdocs-material
+- pip install --user mkdocs==0.17.5 mkdocs-material==2.9.4
 script:
 - mvn test -B
 - |