Platform CI: bundle install should not run in parallel

otherwise, bundle install breaks sometimes like this:

...
Installing rake 12.0.0
Errno::ENOENT: No such file or directory @ realpath_rec -
/var/cache/gitlab-runner/ruby/2.1.0/gems/rake-12.0.0
An error occurred while installing rake (12.0.0), and Bundler cannot continue.
Make sure that `gem install rake -v '12.0.0'` succeeds before bundling.
ERROR: Build failed: exit code 1

https://0xacab.org/varac/platform/pipelines/1371

(from redmine: created on 2016-12-08)