Either get rid of or stabilize go-cty repo fork

[johakoch]

Couper uses the https://github.com/avenga/go-cty fork of https://github.com/zclconf/go-cty. Either

• try to incorporate https://github.com/avenga/go-cty/pull/1 into https://github.com/zclconf/go-cty, or
• stabilize the fork by e.g. regularly updating it and creating a test setup.

[johakoch]

I opened an issue: https://github.com/zclconf/go-cty/issues/156