ci: pin robotframework version to fix the build on TravisCI

When we don't specify robotframework version, robotframework-lint pulls the latest. This makes the
build process less stable as dependencies versions might not work in our environment or conflict
with robotframework-lint itself.

This is exactly what has happened this time: robotframework 4.0.3 pulls ruamel 0.2.4 that requires
Python 3.5 that we don't have on Travis CI (but will be available after #1467).

Fix #1536

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 [email protected];
-      pip install --user html5validator==0.3.3 robotframework-lint==1.1 ansible==2.9.2 ansible-lint==4.2.0;
+      pip install --user html5validator==0.3.3 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;