Fix pip installation on CentOS 6

Previously, the pip install was failing with:
InsecurePlatformWarning: A true SSLContext object is not available.

builder/Dockerfile.centos-6

@@ -50,8 +50,8 @@ RUN yum -y update \
     && yum clean all
 
 # Install pip
-RUN curl "https://bootstrap.pypa.io/2.6/get-pip.py" -o "get-pip.py" && \
-    python get-pip.py
+RUN yum install -y epel-release && \
+   yum install -y python-pip
 
 # Install AWS CLI.
 RUN pip install awscli --upgrade --user && \