Add libffi-dev Ansible 2.x dependency.

docker/build/go-agent/Dockerfile

@@ -42,6 +42,9 @@ RUN apt-get update && apt-get install -y \
     php5-common \
     php5-cli
 
+# Install libffi-dev - a dependency for Ansible 2.x
+RUN apt-get update && apt-get install -y libffi-dev
+
 # Install drush (drupal shell) for access to Drupal commands/Acquia
 RUN php -r "readfile('http://files.drush.org/drush.phar');" > drush && \
     chmod +x drush && \