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 https://github.com/stephenb/sendgrid/commit/425e6c2b4afc890c702bb76f8fdf2e9123b9dce8