Add dependency on parallel_tests

[ekohl]

The rake task release_checks uses the parallel_tests so this should be
either a dependency or the task should fall back on regular tests. Given
parallel_tests works well, I decided on the former.

Note that since parallel_tests depends on ruby >= 2.0 this also means
puppetlabs_spec_helper depends on that.

[puppetcla]

Waiting for CLA signature by @ekohl

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[ekohl]

IMHO this is a trivial patch since it's only a metadata change.