I am a bit new to this, so sorry if there are some missing infos initially.
I installed ruby 1.8.7 via homebrew initially, so I did with shotgun as well.
Then I realized that for a proper ruby+sinatra+shotgun combo, I'll need ruby 1.9.3, so I installed rbenv, and then installed ruby 1.9.3-p0 with it. After that, the ruby -v displays the correct new version, rbenv only lists 1.9.3 and marks it as installed, but when launching shotgun, the info says it is still using 1.8.7. The OS is Lion.
Here's the relevent output
Zsolts-iMac:test zsolt$ shotgun server.rb
== Shotgun/WEBrick on http://127.0.0.1:9393/
[2012-03-01 09:25:49] INFO WEBrick 1.3.1
[2012-03-01 09:25:49] INFO ruby 1.8.7 (2010-01-10) [universal-darwin11.0]
[2012-03-01 09:25:49] INFO WEBrick::HTTPServer#start: pid=73577 port=9393
Zsolts-iMac:teste zsolt$ ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin11.3.0]
Zsolts-iMac:teste zsolt$ rbenv versions
* 1.9.3-p0 (set by /Users/zsolt/Downloads/teste/.rbenv-version)
Zsolts-iMac:teste zsolt$ rbenv version
1.9.3-p0 (set by /Users/zsolt/Downloads/teste/.rbenv-version)
shotgun was missing from the gem list, so I installed it (so possibly the issue was that I installed it via homebrew initially?), and ran a gem update, reloaded the terminal, and now shotgun is using 1.9.3 properly.