Specify gem version only once

[meganemura]

and fix gem version to current rubygems' one.

[thomasrockhu]

This might not have been your original intent, but I think there was some strange dependency going on with Simplecov.

[thomasrockhu]

Thanks for this @meganemura!

[codecov]

Codecov Report

Merging #40 into master will decrease coverage by 1.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   90.47%   88.98%   -1.49%     
==========================================
  Files           1        1              
  Lines         336      336              
==========================================
- Hits          304      299       -5     
- Misses         32       37       +5     

Impacted Files	Coverage Δ
lib/codecov.rb	88.98% <100.00%> (-1.49%)	⬇️

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