Docker uptime isn't fixed

[vishr]

This is in reference to #501. I still see an error 0 strconv.ParseInt: parsing "66518.14": invalid syntax with the following test code:

package main

import (
	"fmt"

	"github.com/shirou/gopsutil/host"
)

func main() {
	t, err := host.Uptime()
	fmt.Println(t, err)
}