IronWorker - execute binary from ruby worker

Output.md

IronWorker log shows this output:

get file from AWS
TODO: Call solver with file at cube.stl
/mnt/task/solver_worker.rb:7:in `exec': Permission denied - solver/sns (Errno::EACCES)
	from /mnt/task/solver_worker.rb:7:in `<top (required)>'
	from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from __runner__.rb:224:in `<main>'
total 12
drwxr-xr-x 3 nobody root 4096 May 17 19:32 .
drwxr-xr-x 4 nobody root 4096 May 17 19:34 ..
drwxr-xr-x 2 nobody root 4096 May 17 19:32 sns
Deleted file

solver.worker

runtime "ruby"
stack "ruby-2.1"
merge_gem 'fog'
file 'sns', 'solver/sns'
full_remote_build true
exec "solver_worker.rb"

solver_worker.rb

# ... code to pull a file from AWS storage and
# save it to the IronWorker environment at `save_location`.
puts "get file from AWS"
begin
  `chmod +x solver/sns`
  puts `ls -la solver/`
  exec('solver/sns', save_location)
ensure
  File.delete(save_location)
  puts "Deleted file"
end

Line 7 of solver_worker.rb should run the sns executable with the filepath at save_location as its argument, but instead it gets a Permission denied - solver/sns (Errno::EACCES) error, as shown in output.md.

I was using this code as a reference