pin rake-compiler to ~> 0.8.3

this avoids the rubygems restriction on >= 1.8.25 that
has been trolling me.  it will also prevent using cross-
compilation as well.  however, the ruby that we ship with
windows chef is old enough that it doesn't satisfy the
rubygems constraint.  until that gets bumped up, we'll
bump down rake-compiler here.
bump version to 0.1.15