[BioC] limma dependency on R 1.8.1 / R 1.9.0

[Gordon Smyth]

The limma package has now forked into two series, one which depends on R 
1.9.0 and one which depends on R 1.7.1-1.8.1.

Limma versions 1.5.0 and higher depend on R 1.9.0 and are in the 
Bioconductor development package area. These versions are developmental and 
are not recommended to users.

Limma versions 1.3.X etc continue to depend on R 1.7.1 - 1.8.1 and are 
being incremented at http://bioinf.wehi.edu.au/limma.

Gordon