Enable installing new version of package before blocker is uninstalled.
Enable installing new version of package before blocker is uninstalled.

BUG=chromium-os:17560
TEST=Verify parallel_emerge can handle blockers uninstalling after pkg
     is installed.

Change-Id: I14c88990747a7360b400bd430e969022cac21c33
Reviewed-on: http://gerrit.chromium.org/gerrit/4038
Reviewed-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite