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

This closes #1158
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 \