Remove unnecessary dependency for ruby standard gem

[uplus]

I want to reduce unnecessary dependency from my product's Gemfile.lock.
The json gem is contained in the ruby standard gems.
They get automatically installed when you install Ruby.

The following Gemfile.lock is minimum project example that using codecov.

Gemfile.lock (before this PR)

GEM
  remote: https://rubygems.org/
  specs:
    codecov (0.2.12)
      json
      simplecov
    docile (1.3.2)
    json (2.3.1)
    simplecov (0.19.0)
      docile (~> 1.1)
      simplecov-html (~> 0.11)
    simplecov-html (0.12.3)

PLATFORMS
  ruby

DEPENDENCIES
  codecov

BUNDLED WITH
   2.1.4

Gemfile.lock (after this PR)

GEM
  remote: https://rubygems.org/
  specs:
    docile (1.3.2)
    simplecov (0.19.0)
      docile (~> 1.1)
      simplecov-html (~> 0.11)
    simplecov-html (0.12.3)

PLATFORMS
  ruby

DEPENDENCIES
  codecov

BUNDLED WITH
   2.1.4

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files           1        1           
  Lines         356      356           
=======================================
  Hits          327      327           
  Misses         29       29           

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[thomasrockhu]

Thanks @uplus !