binding ruby: Bundle glib-1.2.6 to build milter manager with Ruby1.8.7
binding ruby: Add pkg-config.rb to find pkgconfig files
configure: Support Ruby1.8.7 again
Revert "travis: Try to add Ruby1.8.5"

This reverts commit 09b1f39db8fa9d3c399c56576ea240fad014f44e.
Because TravisCI doesn't support Ruby1.8.5.
travis: Add Ruby1.8.7