Remove duplicate dependencies

[diazruy]

Getting a warning from Bundler otherwise

The gemspec at /home/ruy/.rvm/gems/ruby-2.3.0/bundler/gems/sendgrid-e1bdb3170968/sendgrid.gemspec is not valid. The validation error was 'duplicate dependency on json (>= 0), (>= 0) use:
    add_runtime_dependency 'json', '>= 0', '>= 0'

Not very familiar with gem builing, but I believe this was added in error in 425e6c2

[markbluhm]

+1 and in bundler 2.7.1 I'm getting

Bundler found incorrect dependencies in the lockfile for sendgrid-1.2.4

granted, our team needs to move to the sendgrid-ruby gem, I thought I should at least leave this comment

2.6.9 did it too, just went back to the 2.5.6 that was working before.

[andrii-baran-nfgp]

+1 with bundler 2.7.2