chore: install ruby-bundler
Fixes recent build failures in the Travis CI environment.

This closes #1158
diff --git a/build/docker/debian/Dockerfile b/build/docker/debian/Dockerfile
index 99eda7a..7c15109 100644
--- a/build/docker/debian/Dockerfile
+++ b/build/docker/debian/Dockerfile
@@ -83,6 +83,8 @@
       libio-socket-ssl-perl \
       libnet-ssleay-perl
 
+RUN echo "deb http://ftp.debian.org/debian jessie-backports main" > /etc/apt/sources.list.d/jessie-backports.list
+RUN apt-get update && apt-get -t jessie-backports install -y ruby-bundler
 RUN apt-get update && apt-get install -y --no-install-recommends \
 `# Php dependencies` \
       php5 \
diff --git a/build/docker/ubuntu/Dockerfile b/build/docker/ubuntu/Dockerfile
index 7792aae..02f0b1e 100644
--- a/build/docker/ubuntu/Dockerfile
+++ b/build/docker/ubuntu/Dockerfile
@@ -95,6 +95,7 @@
 `# Ruby dependencies` \
       ruby \
       ruby-dev \
+      ruby-bundler \
 `# Perl dependencies` \
       libbit-vector-perl \
       libclass-accessor-class-perl \