[all] Correctly identify Windows on JRuby
[all] Correctly identify Windows on JRuby

* Identifying Windows correctly is essential for ensuring various Rake
  tasks work as expected, implicit sudo is not used and the right
  commands are passed to cmd.exe. This patch should help with install
  problems running JRuby on Windows.

Signed-off-by: Alex Coles <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@alexcolesportfolio.com>

datamapper/dm-adjust