Fix invalid download paths preventing install

Invalid download paths with double 'go' prefixes were introduced in #84, breaking installs completely. Most importantly, the breakage prevents 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 https://go.googlecode.com does not).

I've also taken the liberty to fix tab indentation where inconsistent.