Build error with older golang version.

[prataprc]

$ go version
go version go1.10.4 linux/amd64

$ go build
# github.com/FastFilter/xorfilter
/home/prataprc/myworld/devgo/src/github.com/FastFilter/xorfilter/xorfilter.go:54:9: invalid operation: n << (c & 63) (shift count type int, must be unsigned integer)
/home/prataprc/myworld/devgo/src/github.com/FastFilter/xorfilter/xorfilter.go:54:27: invalid operation: n >> (-c & 63) (shift count type int, must be unsigned integer)

Note: Build is okay with latest golang version.

[lemire]

Thanks.