Fix invalid download paths preventing install

Invalid download paths with double 'go' prefixes were introduced in #84, breaking installs completely for me. Most importantly, this prevents my Travis builds from even starting.

These commits fix the path generation, as well as change the fallback download URL to use https://storage.googleapis.com/golang, which does actually still work (while go.googlecode.com does not).

I've also taken the liberty to fix tab indentation where inconsistent.