Update Go in CI build.

Travis OSX images ship with an older version of Go (1.3.1) than their normal Ubuntu ones, making the build fail on a `go get`, this updates it to Go 1.3.3. Same goes for AppVeyor.

Leaving a request for Travis and AppVeyor to update their environment would be ideal, but for now this works.
The Travis build will also only update if it is outdated.
Still some failures on the Travis build, but they are unrelated to this.