ci: downgrade and pin robotframework to unbreak rflint.

rflint doesn't work with robotframework >= 3.2.0 and fails with "ImportError: No module named
txtreader" exception. Until fix is available, downgrade robotframework to the previous working
version (3.1.2)

Workaround for boakley/robotframework-lint#73

Fix #1404

Author: php-coder

.travis.yml

@@ -21,7 +21,7 @@ before_script:
       mkdir -p /tmp/uploads /tmp/preview;
       cp src/main/resources/test/test.png /tmp/uploads/1.png;
       npm install -g bootlint;
-      pip install --user html5validator==0.3.1 robotframework-lint ansible==2.9.2 ansible-lint;
+      pip install --user html5validator==0.3.1 robotframework-3.1.2 robotframework-lint ansible==2.9.2 ansible-lint;
       if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
         gem install danger nokogiri --no-ri --no-rdoc;
       fi;