Bug 656736 - Support upgrading to the latest version of MozMill. r=Standard8
This patch adds mozmill 1.5.4b4, mozrunner 2.5.5b4 and jsbridge 2.4.4b4 to the
tree. It also sets up a virtualenv to install mozmill and its dependencies
in. This patch means Thunderbird developers will no longer need to have MozMill
or its dependencies installed in order to run MozMill tests.