Failed to activate imath

[Joshua Root]

> --->  Activating imath @3.1.12_0
> Error: Failed to activate imath: The following ports have active files that conflict with imath's:
> ilmbase @2.3.0_1
>    /opt/local/lib/libImath.dylib

This is <https://trac.macports.org/ticket/68111>. According to one of 
the imath maintainers in the ticket, it only happens if you haven't 
upgraded in over a year.

Running 'sudo port migrate' may be the most reliable way to upgrade a 
very old installation.

- Josh