Tweak python client install

Author: jbrisbin

README.adoc

@@ -122,12 +122,11 @@ endif::riak_explorer[]
 ifdef::riak_client[]
 # Install the Python client
 ifdef::ubuntu,debian[]
-RUN apt-get install -y build-essential libssl-dev
-RUN apt-get install -y python-pip python-dev
+RUN apt-get install -y build-essential libssl-dev libffi-dev python-dev python-pip
 endif::[]
 ifdef::centos[]
 RUN yum groups install -y 'Development Tools'
-RUN yum install -y openssl-devel python-devel python-pip
+RUN yum install -y openssl-devel python-devel libffi-devel python-pip
 endif::[]
 RUN pip install --upgrade pip cryptography riak
 endif::riak_client[]