Add Go Versions up to 1.12 to Travis config

[robbieheywood]

Versions of relevant software used Go versions from 1.8 up to 1.12

What happened We should add Go versions from 1.8 to 1.12 to the travis config so that these are covered by CI.

What you expected to happen Travis CI runs for Go versions from 1.7 - 1.12.

How to reproduce it (as minimally and precisely as possible): All PRs

Full logs to relevant components n/a

[bwplotka]

:+1: