Update pip on every build

Docker/lofar-ci/Dockerfile_ci_base

@@ -8,6 +8,7 @@ FROM centos:centos7.6.1810
 RUN yum -y groupinstall 'Development Tools' && \
     yum -y install epel-release && \
     yum -y install cmake cmake3 gcc git log4cplus-devel python3 python3-devel python3-pip which wget curl atop valgrind && \
+    python3 -m pip install -U pip && \
     pip3 install kombu==4.6.8 requests coverage python-qpid-proton && \
     adduser lofarsys && \
     mkdir -p /opt/lofar && chown -R lofarsys:lofarsys /opt