There was a recent commit to Sinatra that changed the default listen address to localhost from 0.0.0.0 in development mode due to security concerns.
In order to explicitly allow access from the network, you need to either run your app in another mode (e.g. production), or set the bind option to 0.0.0.0.
You can do this from the command line using the built in server using the -o option:
$ ./my_sinatra_file.rb -o 0.0.0.0