Remove explicit parallel_test gem requirement

Your documentation shows to install parallel_test. However, doing so installs vs 1.0.3, which then fails because sauce requires 0.16.6. Here is the error:
```
Bundler could not find compatible versions for gem "parallel_tests":
  In Gemfile:
    sauce (= 3.4.9) ruby depends on
      parallel_tests (= 0.16.6) ruby

    parallel_tests (1.0.3)
```
Removing the explicit parallel_tests requirement makes everything work fine.