Remove go version 1.6, add 1.8 and 1.9 to match zap
Remove go version 1.6, add 1.8 and 1.9 to match zap

Update travis ci config such that supported go versions match the
versions supported by github.com/uber-go/zap library.

Zap gives a `undefined: runtime.CallersFrames` error in go 1.6.
See https://github.com/uber-go/zap/issues/389

uber-common/bark