Update gem options in Travis build

[altendky]

rubygems/rubygems#2354

Change noticed while working #103.

[codecov]

Codecov Report

Merging #104 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #104   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          446       446           
  Branches        69        69           
=========================================
  Hits           446       446           

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[altendky]

@python-desert this blocks all other PRs from passing. Any concerns about this?

[python-desert]

I don't really know what this is but okay

[altendky]

Neither do I but a quick look makes it seem like we use the travis lint tool to lint the CI config in CI and this is in Ruby and gem deprecated the options we have been using. So it's just a catch up with the change to the gem command line options.