Upgraded go version in the Dockerfile for bosun build

[grzkv]

The Bosun compilation by running bosun/cmd/bosun/docker/docker.sh currently fails:

> ./docker.sh
> ...
> Step 14/14 : RUN go run build/build.go     && bosun -version     && scollector -version     && tsdbrelay -version
> building bosun
> go install -ldflags -X bosun.org/_version.VersionSHA=03ad93d1efaf9a14d95f9a8da7e9cf08974ae67c -X bosun.org/_version.VersionDate=20181215204533 bosun.org/cmd/bosun
# bosun.org/cmd/bosun/database
cmd/bosun/database/migrate.go:109: undefined: sort.Slice
2018/12/15 20:46:33 exit status 2
exit status 1

This happens because the build is using go1.7, while sort.Slice was introduced in 1.8, see https://golang.org/pkg/sort/#Slice

This commit contains the golang version upgrade 1.7 → 1.8 needed for compilation to proceed.

@kylebrandt @captncraig please have a look.

Perhaps, it makes sense to upgrade to an even higher version, not just to 1.8.

[grzkv]

According to #2319 will upgrade to v1.11. Thanks, @pradeepbbl for pointing this out.

[grzkv]

Updated version as discussed. Please, merge if no objections.