chore: omit spec/ from .gem file

[mroach]

The test_files attribute was already being set in twilio-ruby.gemspec, but this does not prevent the test files from being added to the .gem bundle and installed onto the system/image.

Prevent spec/test files from being added to the bundle in the first place by adding a reject filter. This practice is now the default when creating new gems: https://github.com/rubygems/rubygems/blob/85677a06a727d5053e9f4a3f13355775ed1e0fd5/bundler/lib/bundler/templates/newgem/newgem.gemspec.tt#L27-L31

This change shrinks the disk space used by about ~4.5 MB:

du -sh /opt/local/lib/ruby/gems/2.7.0/gems/twilio-ruby-5.*
13M     /opt/local/lib/ruby/gems/2.7.0/gems/twilio-ruby-5.48.0
8.6M    /opt/local/lib/ruby/gems/2.7.0/gems/twilio-ruby-5.50.0

Fixes

A short description of what this PR does.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
  I don't believe there is a realistic way to test this. I tested it manually, if that counts.
• I have added the necessary documentation about the functionality in the appropriate .md file
  I don't believe this applies
• I have added inline documentation to the code I modified
  I don't believe this is required.

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[JenniferMah]

LGTM! Thanks for the contribution!