add the parallel_tests gem

[bastelfreak]

this allows us to run spec tests in parallel. We already enabled this in
our configs in the past, but we didn't include this gem. It was a
dependency of puppetlabs_spec_helper. This got removed so we need to
install it explicitly. Changes tested at:
https://github.com/voxpupuli/puppet-zabbix/blob/master/Gemfile#L39
https://travis-ci.org/voxpupuli/puppet-zabbix/builds/235141105

[domcleal]

Could you also change the rspec output format to dots in that case, in moduleroot/.rspec.erb or .rspec_parallel.erb if you want to only change the parallel mode?

It should make the output much easier to read than the documentation style when there's a failure, otherwise the output becomes interleaved between the parallel processes and it's very hard to find the error.

[domcleal]

Thanks @bastelfreak.