Drop cross-compilation of FreeBSD binary artifacts (#84)

There were too many compatibility problems with some dependencies due to
native bindings. Therefore only builds for the main OS and architectures
(Linux, macOS and Windows) are compiled for now which covers almost all
users.

Epic GH-33
Resolves GH-83

magefile.go

@@ -75,7 +75,6 @@ var (
 	// Also see the source of the `build` command: https://github.com/golang/go/blob/master/src/cmd/dist/build.go
 	crossCompileTargetPlatforms = []string{
 		"darwin/amd64",
-		"freebsd/amd64",
 		"linux/amd64",
 		"windows/amd64",
 	}