Update hl7 dependency

[ransford]

Go dependencies have become easier to fetch and manage thanks to modules and the default global proxy. This PR

• Makes the minimal switch to go mod for dependencies
• Removes the dependency on the virtalabs/hl7 mirror of fknsrs.biz/p/hl7, as "module host is down" (the original motivation for that mirror) is no longer a real concern for Go modules thanks to the global module proxy hosted by Google