Switch to #!/usr/bin/env ruby

[pierre-24]

If, for some reasons, there is no /usr/bin/ruby (... Because it is located somewhere else), then auto-generation of tests with generate_test_runner.rb (used directly as an executable) fails because of ``#!/usr/bin/ruby. Thus, I propose to use #!/usr/bin/env ruby` instead :)

More context for those interested: on a supercomputer, more or less nothing (except the very basic commands) is at its standard location, but rather loaded on the fly using modules ;)