Use Ruby included with Sonic Pi for start-server

Sonic Pi's server/bin/sonic-pi-server.rb has a shebang line for #!/usr/bin/env ruby. When running sonic-pi-tool start-server from the command line, it uses whatever ruby it finds in your PATH. If it's not 2.3 (the version bundled with Sonic Pi), it fails:

$ ruby --version
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin16]

$ sonic-pi-tool start-server
/Users/chris/Applications/Sonic Pi.app/app/server/vendor/fast_osc-0.0.11/lib/fast_osc.rb:22:in `require_relative': cannot load such file -- /Users/chris/Applications/Sonic Pi.app/app/server/rb-native/osx/2.4.0/fast_osc (LoadError)
        from /Users/chris/Applications/Sonic Pi.app/app/server/vendor/fast_osc-0.0.11/lib/fast_osc.rb:22:in `rescue in <top (required)>'
[omitted]

It's looking for files in Sonic Pi.app/app/server/rb-native/osx/2.4.0 instead of Sonic Pi.app/app/server/rb-native/osx/2.3.0.

It would be super cool if the start-server command, after determining the correct path to the Sonic Pi installation, started the server using the Ruby included in the package (Sonic Pi.app/app/server/native/osx/ruby/bin).

Again, as mentioned in #17, I'd gladly implement this but I don't have the Rust knowledge to make it happen :(

Locate the Rust implementation of the start-server command and the logic that determines the Sonic Pi installation path. Check how it currently launches server/bin/sonic-pi-server.rb, then verify that it uses the packaged executable under Sonic Pi.app/app/server/native/osx/ruby/bin rather than ruby from PATH. Done means sonic-pi-tool start-server selects the bundled Ruby and avoids the version-specific LoadError.