ci: remove pinning of ruamel.yaml.clib version as it's not necessary anymore

Since 0e09fac commit we have Python 3.6 installed.

Relate to #1536

Author: php-coder

.travis.yml

@@ -25,7 +25,12 @@ before_script:
       sudo apt-get -qq update;
       sudo apt-get install -y shellcheck python3.6 python3-pip;
       curl https://bootstrap.pypa.io/pip/3.6/get-pip.py | sudo -H python3.6;
-      pip3 install --user html5validator==0.4.2 ruamel.yaml.clib==0.2.2 robotframework==3.2.2 robotframework-lint==1.1 ansible==2.9.2 ansible-lint==4.2.0;
+      pip3 install --user \
+        html5validator==0.4.2 \
+        robotframework==3.2.2 \
+        robotframework-lint==1.1 \
+        ansible==2.9.2 \
+        ansible-lint==4.2.0;
       if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
         gem install danger:8.2.2 nokogiri:1.10.10 --no-document;
       fi;