Use specific M2E P2 repository to run M2E 1.4 in Indigo

Indigo is no longer officially supported by M2E team [1], but they've
provided a P2 repository that will allow the installation of M2E 1.4
into Indigo.

[1] http://dev.eclipse.org/mhonarc/lists/m2e-users/msg04198.html