Update TracisCI for Go 1.4 release.

Our policy is to only ensure the code compiles for the latest Go release
minus one version.  Since Go 1.4 has now been released, this commit
updates TravisCI to remove Go 1.2 from the build matrix and add Go 1.3.

Also, go vet changed locations in between Go 1.3 and Go 1.4, so a check
has been added to download the appropriate version based on which version
of Go is running the integration tests.

Finally, the previous removed Go tip entry in the build matrix has been
reintroduced since it will now work again.