Take advantage of bundler's parallelism

20 jobs is somewhat arbitrary.

Parallelism need not be limited to the number of logical cores. Because
installing gems is such an IO-intensive task, generally not CPU-bound,
we can typically see big speed wins by starting many at once.

Dockerfile

@@ -7,4 +7,4 @@ RUN curl -sL https://deb.nodesource.com/setup_4.x | bash - && \
 WORKDIR /exercism
 
 COPY Gemfile Gemfile.lock /exercism/
-RUN bundle install
+RUN bundle install --jobs=20