x/tools/go/gcimporter: update export data format version

[griesemer]

Tracking issue.

x/tools/go/gcimporter15/bexport.go still generates export data format version 1, but now that we can export aliases, we should probably update to the latest version. This is not urgent, but may avoid confusion down the road.

[griesemer]

cc: @alandonovan

[alandonovan]

It is somewhat urgent. For example, Google's source code tools must understand Go 1.8 source and export data before we can start to evaluate 1.8rc1, which will ideally be a long time before the 1.8 release.

[griesemer]

We should still do this, even w/o aliases. We should have all importers understand the latest version.

[gopherbot]

CL https://golang.org/cl/33111 mentions this issue.

[gopherbot]

CL https://golang.org/cl/33113 mentions this issue.