is not support for go mod

[m1zzx2]

run:

 ../scaffold/scaffold/scaffold init
````

will see: 
````
panic: cannot find $GOPATH environment variable

goroutine 1 [running]:
github.com/catchplay/scaffold/scaffold.init.0()
	/Users/zhihu/private_project/scaffold/scaffold/scaffold.go:21 +0x99
````

except: 
````
can generate code in any path
````