(#21043) Use ruby.exe instead of rubyw.exe

Previously, if you set a custom runinterval setting in puppet.conf, puppet
running on ruby 1.9 would ignore it, whereas it would work in ruby 1.8.

The root cause is due to http://bugs.ruby-lang.org/issues/7239. In ruby 1.9,
rubyw.exe is unable to read the stdout of any child process using ruby's
built-in process methods, e.g. %x. This affects us, because the windows
service, running as rubyw.exe, executes:

    `%x{ puppet agent --configprint runinterval }`

to obtain the current runinterval setting.

The bug is fixed in ruby 2.0, so for now, we just switch to using
`ruby.exe` instead of `rubyw.exe`.