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]

👍