Suppress error output when evaluating invalid Ruby during tests

[deivid-rodriguez]

Before:

[dependabot-core-dev] ~/dependabot-core/bundler $ rspec -e "with invalid Ruby"
Run options: include {:full_description=>/with\ invalid\ Ruby/}

Randomized with seed 12398
(string):2:20: error: unexpected token tUMINUS
(string):2:     ruby -rubygems -e 'require "jekyll-import";
(string):2:                    ^                           
.(string):2:20: error: unexpected token tUMINUS
(string):2:     ruby -rubygems -e 'require "jekyll-import";
(string):2:                    ^                           
.(string):2:20: error: unexpected token tUMINUS
(string):2:     ruby -rubygems -e 'require "jekyll-import";
(string):2:                    ^                           
.(string):2:20: error: unexpected token tUMINUS
(string):2:     ruby -rubygems -e 'require "jekyll-import";
(string):2:                    ^                           
.

Finished in 0.04749 seconds (files took 1.39 seconds to load)
4 examples, 0 failures

Randomized with seed 12398

After:

[dependabot-core-dev] ~/dependabot-core/bundler $ rspec -e "with invalid Ruby"
Run options: include {:full_description=>/with\ invalid\ Ruby/}

Randomized with seed 3840
....

Finished in 0.04951 seconds (files took 1.26 seconds to load)
4 examples, 0 failures

Randomized with seed 3840