upgrade to go 1.12

[slingamn]

v1.0.0 should use golang 1.11.5. But we should switch to golang 1.12 for v1.1.0. The only code change is to update travis:

https://github.com/oragono/oragono/blob/234d011c297cbe5f38ee987d86401f638d92b87b/.travis.yml#L4

[csmith]

The docker version is apparently building against 1.12rc1 at the minute. We should probably update that at the same time (and then keep them in sync)