Drop support for go 1.8 from TravisCI builds

[cristim]

The Travis builds are failing on Go 1.8, and this is shown on the Github badge as a build failure.

The failures are because one of the dependencies seems to need type aliasing support available only as of Go 1.9: https://travis-ci.org/Microsoft/vscode-go/builds/444157465

[ramya-rao-a]

Thanks for the PR @cristim

I did log an upstream bug for this in gocode this morning. mdempsky/gocode#73

We still have lots of users on Go 1.8. So if we can get an upstream fix for this issue, I would prefer that to stopping support for Go 1.8 altogether.

Let's wait and see if we get a response upstream

cc @stamblerre

[ramya-rao-a]

@stamblerre brings up a good point in mdempsky/gocode#73 (comment). We can use nsf/gocode for Go versions < 1.10

@cristim Would you be interested in looking into that?

Code pointers:

• Build and Debug extension to have a local set up of this extension up and running
• Instead of directly accessing allTools in goInstallTools.ts, have a function that accepts tool and goVersion. This way we can use the nsf/gocode for older versions of Go

[ramya-rao-a]

I've made the changes to fallback to nsf/gocode for Go 1.8