Fix CI_SPEC_OPTIONS

[stamm]

If I run env CI_SPEC_OPTIONS="-n3 --serialize-stdout" bundle exec rake parallel_spec I got an error:

4 processes for 81 specs, ~ 20 specs per process
invalid option: -3

Please use --help for a listing of valid options
invalid option: -3

Please use --help for a listing of valid options
invalid option: -3

Please use --help for a listing of valid options
invalid option: -3

Please use --help for a listing of valid options

I printed args:

"-t",
 "rspec",
 "--",
 "-n3",
 "--serialize-stdout",
 "--",
 "spec/classes/profiles/docker/config_spec.rb",
…

I think we don't need to push -- elements

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ef83ccd). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master       #1   +/-   ##
=========================================
  Coverage          ?   40.34%           
=========================================
  Files             ?       10           
  Lines             ?      751           
  Branches          ?        0           
=========================================
  Hits              ?      303           
  Misses            ?      448           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/puppetlabs_spec_helper/rake_tasks.rb	41.46% <0%> (ø)

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