[Fix #3278] Lock json version for 1.9.3 builds (#3290)
[Fix #3278] Lock json version for 1.9.3 builds (#3290)

Recently the `json` gem was updated to require Ruby >= 2.0. This
causes our Ruby 1.9.3 build to fail. Here we lock the `json` gem
to the previously working version for 1.9 builds.
jonas054/rubocop,jonas054/rubocop