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.
Yes. As I mentioned in the PR, https://go.googlecode.com/files has been retired and is now returning 404s (see: https://go.googlecode.com/files/go1.3.darwin-amd64-osx10.8.pkg). Instead of removing the fallback entirely, it seemed better to at least point to https://storage.googleapis.com/golang, since it seems to still be serving downloads.