Remove executables from gemspec

[hugopeixoto]

This gem does not have any executables. Previous versions included the rubocop/rspec/appraisals binstubs. Fixes #58

Tests before the PR:

shortcode$ bin/rspec
The `rspec` executable in the `rspec-core` gem is being loaded, but it's also present in other gems (shortcode).
If you meant to run the executable for another gem, make sure you use a project specific binstub (`bundle binstub <gem_name>`).
If you plan to use multiple conflicting executables, generate binstubs for them and disambiguate their names.

Randomized with seed 65244
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Finished in 0.20697 seconds (files took 0.5488 seconds to load)
59 examples, 0 failures

Randomized with seed 65244

Tests after the PR:

$ bin/rspec

Randomized with seed 43142
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Finished in 0.2537 seconds (files took 0.53934 seconds to load)
59 examples, 0 failures

Randomized with seed 43142