Can't build with Go 1.15

[DamienCassou]

I tried building grobi 0.6.0 with Go 1.15 but got:

./randr.go:224:19: conversion from uint16 to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL    github.com/fd0/grobi [build failed]

[DamienCassou]

Because Go 1.14 is not supported anymore, I suggest releasing a new version of grobi with the latest patch including a fix for the issue I report above.