layers.txt: meta-ros commit updated.

Some users are experiencing difficulties when bitbaking
the recipes because the current commit is not updated.
I'm proposing commit 1a6a63e
which points to the last release (v0.1) of meta-ros.
Merge pull request #29 from vmayoral/angstrom-v2012.12-yocto1.3

layers.txt: meta-ros commit updated.
Merge remote-tracking branch 'origin/angstrom-v2012.12-yocto1.3'