Do not require `v` prefix on Gleam versions

[lpil]

# beamup install gleam 1.6.2
Created a default config at "/root/.config/beamup/config.toml"
Downloading and installing Gleam for release=1.6.2 id=1.6.2
Error: Failed fetching Github release 1.6.2 from gleam-lang/gleam

Caused by:
   0: GitHub
   1: Not Found
      Documentation URL: https://docs.github.com/rest/releases/releases#get-a-release-by-tag-name

Location:
    src/github.rs:142:29

[tsloughter]

Yea, this was a bit of laziness as well as the thinking that accepting a "version" implied more verification than beamup was going to do. So I left it as taking a release name.

Probably for the best if after a Not Found is return that it first makes an attempt by adding v.

[tsloughter]

I can still be lazy and even check for vOTP-missing, hehe.