Pin RuboCop version for Travis tests

[JacobEvelyn]

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Hi there! Thanks so much for submitting a pull request!

Let's just make sure together that all of these boxes are checked before we
merge this change:

• The code in these changes works correctly.
• Code has tests as appropriate.
• Code has been reviewed by @JacobEvelyn.
• All tests pass on Travis CI.
• Code coverage remains high.
• Rubocop reports no issues on Travis CI.
• The README.md file is updated as appropriate.

Don't worry—this list will get checked off in no time!

[codecov]

Codecov Report

Merging #234 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master    #234   +/-   ##
======================================
  Coverage    98.3%   98.3%           
======================================
  Files          23      23           
  Lines         709     709           
======================================
  Hits          697     697           
  Misses         12      12

Impacted Files	Coverage Δ
lib/friends/introvert.rb	98.58% <0%> (ø)	⬆️

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