comm-central and comm-aurora Linux builders switched to gcc 4.5

[Mark Banner]

Following Firefox's move last month, we've now switched out Linux
builders on comm-central and comm-aurora to gcc 4.5.

This resolves the persistent leak test failures that we were getting on
trunk. The Linux builders are still recovering from their associated
clobber, but are looking good so far.

For those using Linux nightlies, please keep an eye out for any issues
and file bugs in the normal place.

Mark.