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 (thanks to the user 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).
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