Updated to use go modules

[tapirs]

removed the vendor directory and initiated as a module
updated tfjson_test
Updated circle ci config
Added env variable to turn on modules
added env variable to test run and added names to runs
updated env variable
installed terraform
downloading dependencies before building terraform

[nmiyake]

Thanks for the PR!

I've updated the project to use CircleCI 2.0, so it should be possible to update/touch up your PR and have the jobs run. Once this is running in CI I can provide feedback on the actual PR.

[tapirs]

Hi I have updated my PR to pass all of the tests, the only problem is go version 1.10 doesn't support modules and so the vendor directory has to be included so implementing modules then becomes pointless as the dependencies are satisfied within the repo.

Would it be better to create a release containing the vendor director for pre 1.11 go and then test only using version 1.11 and version 1.12 after that?