(maint) sort dependencies in gemspec

[DavidS]

From robocop 0.51

[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #6   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files           7        7           
  Lines         281      281           
=======================================
  Hits          275      275           
  Misses          6        6

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[coveralls]

Coverage remained the same at 97.865% when pulling 6cb563e on DavidS:rubocop-fixes into d7d5f3d on puppetlabs:master.