THRIFT-3696 Install pip to CentOS Docker images to fix Python builds

This closes #912
diff --git a/build/docker/centos/Dockerfile b/build/docker/centos/Dockerfile
index 4ce4f42..011ccc0 100644
--- a/build/docker/centos/Dockerfile
+++ b/build/docker/centos/Dockerfile
@@ -58,7 +58,7 @@
       python-devel \
       python-setuptools \
       python-twisted-web \
-      python-six
+      python-pip
 
 # Ruby Dependencies
 RUN yum install -y \