FindRuby: add "ruby1.9.1" to list of possible ruby-interpreter

not sure if this is actually a good fix...

got this because "tools/orogen_metadata" didn't specify
"-DRUBY_EXECUTABLE=/usr/bin/ruby1.9.1" on the cmake-line (as _some_
projects do) and thus couldn't find any ruby-interpreter with the
current FindRuby.cmake -- hence this fix.

- wasn't there this executable file named "ruby" in the PATH, just
  calling the correct "/usr/bin/ruby1.9.1", removing the need for any
  ruby-lookup?
- which is the preferred ruby-version? 1.9.1 (which is actually version
  1.9.3, as ruby1.9.1 reports on my Debian...) is quite old, 2.1.2 is
  there in the moment. I assume mixing is not possible...
- why do some projects give the ruby-interpreter to cmake already, and
  some not? non-consistent stuff is bound to break...

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>