Ensure compatibility with crystal 0.27.0

[waghanza]

Hi,

According to https://github.com/crystal-lang/crystal/blob/master/CHANGELOG.md, there is some breaking change in crystal.

This PR is a starting point for compatibility.

Tempfile does not exists anymore, file.tempfile is used instead
crystal-lang/crystal#6485

Regards,

[waghanza]

@faustinoaq for me the next error is due to crystal-lang/crystal#6514, but I don't if this is from teeplate or from amber

[drujensen]

@waghanza thanks for this. I opened a new PR that includes all the needed changes #973. We are waiting for a Crecto update for the final specs to pass.