Switch to dep for dependency management

vendor/github.com/mattn/go-isatty/isatty_appengine.go

@@ -7,3 +7,9 @@ package isatty
 func IsTerminal(fd uintptr) bool {
 	return false
 }
+
+// IsCygwinTerminal() return true if the file descriptor is a cygwin or msys2
+// terminal. This is also always false on this environment.
+func IsCygwinTerminal(fd uintptr) bool {
+	return false
+}