Remove golang.org/x/net/internal/iana dependency
Remove golang.org/x/net/internal/iana dependency

Since Go 1.5, internal package rule (see
https://golang.org/s/go14internal for more details) applies to GOPATH
packages. It prevents this package from loading iana package and causes
package building failure.

This fixes it by adding its own ICMP protocol constants and removing
iana package dependency.

Fix #10

tatsushid/go-fastping,urtho/go-fastping,kanocz/go-fastping