min-required should support json files, not just *.tf files

[jrobison-sb]

Terraform configuration files are most commonly done in HCL and with a *.tf extension, but they also support JSON in 0.12 and 0.11.

It looks to me like tfenv install min-required is only parsing *.tf files, not JSON. min-required should probably support the same file formats as Terraform itself supports, so JSON support would be nice.

[Zordrak]

PR welcomed

[sebglon]

I submitted a very quick PR #277 to improve this part. Thank's for a review