jenkins: limit to pylint < 1.8

Newest version is broken (see build 1762 for example)

jenkins.sh

@@ -12,7 +12,7 @@ export PIP_USE_WHEEL=no
 (pip freeze | grep -q Quixote) || pip install quixote
 (pip freeze | grep -q scgi) || pip install scgi
 pip install --upgrade 'pip<8'
-pip install --upgrade pylint pylint-django
+pip install --upgrade 'pylint<1.8' pylint-django
 pip install --upgrade -r requirements.txt
 pip install --upgrade pytest pytest-django pytest-cov WebTest django-webtest 'setuptools<34'
 pip install --upgrade 'django>=1.8, <1.9'