ast: we don't need zero node

Also trigger a build on Travis

ast/ast.go

@@ -10,12 +10,6 @@ type Node interface {
 	Pos() token.Pos
 }
 
-// NewNode returns a non usable Node interface implementer. The position is
-// initalizied to zero.
-func NewNode() Node {
-	return &zero{}
-}
-
 func (File) node()       {}
 func (ObjectList) node() {}
 func (ObjectKey) node()  {}
@@ -27,14 +21,6 @@ func (ObjectType) node()   {}
 func (LiteralType) node()  {}
 func (ListType) node()     {}
 
-type zero struct{}
-
-func (zero) node() {}
-
-func (z *zero) Pos() token.Pos {
-	return token.Pos{}
-}
-
 // File represents a single HCL file
 type File struct {
 	Node     Node            // usually a *ObjectList