ci: install Python 3.6 that is required by newer version of html5validator to fix the build on TravisCI

This won't be necessary once we get newer Ubuntu on CI (see #1467)

See also:
- https://askubuntu.com/questions/865554/how-do-i-install-python-3-6-using-apt-get
- https://askubuntu.com/questions/889535/how-to-install-pip-for-python-3-6-on-ubuntu-16-10

Correction for 0132a12 commit.

Relate to #1533

Author: php-coder

Diff for: .travis.yml

@@ -21,12 +21,14 @@ 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==0.4.2 ruamel.yaml.clib==0.2.2 robotframework==3.2.2 robotframework-lint==1.1 ansible==2.9.2 ansible-lint==4.2.0;
+      sudo add-apt-repository -y ppa:jblgf0/python;
+      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;
       if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
         gem install danger:8.2.2 nokogiri:1.10.10 --no-document;
       fi;
-      sudo apt-get -qq update;
-      sudo apt-get install -y shellcheck;
     elif [ "$SPRING_PROFILES_ACTIVE" = 'postgres' ]; then
       psql -U postgres -c 'create database mystamps;';
     fi