Fix vercomp() in goenv-install, update download URLs.

The `vercomp()` function in libexec/goenv-install had a number of issues, most notably related to attempting installation of Go versions with three version numbers. Replaced the previous `vercomp()` with a slightly adapted version [available from Stack Overflow](http://stackoverflow.com/questions/3511006) (thanks to the user [fgm](http://stackoverflow.com/users/57457/fgm)).

The download URL for more recent versions of Go has also changed once again. In addition to this, the download URL for Go 1 differs from the URLs for all other versions of Go, which has been corrected (admittedly, in a hackish way).

I also took the liberty of amending the tag referenced in the installation section of README.md, as it erroneously refers to the non-existent 'v0.0.5' tag (if desired -- this was isolated to its own commit).