Cache bundler directory in Travis

[JacobEvelyn]

Closes #260

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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:

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

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

[codecov[bot]]

Codecov Report

Merging #261 into master will not change coverage by %. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   98.41%   98.41%           
=======================================
  Files          24       24           
  Lines         755      755           
=======================================
  Hits          743      743           
  Misses         12       12           

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