Communicate to user when we skip a requirement

We skip several requirements on python >= 2.6 because they
are only needed on 2.6 but there is no good way to express that in
requirements.txt. This can be confusing to users, because they may
not be expecting such a thing and may think something broke.

Print a message to the info log to that the user can see that we
chose to not install their non-needed requirement.

Change-Id: I11de12351123bbf406f77cf402feeab0e5dff26b
Merge "Communicate to user when we skip a requirement"