vegan depends on R (>= 3.3.0) which added generic sigma()

reported as github issue #274. We failed to check vegan with
older version of R and gave false promise of 3.1.0 compatibility.

I could not check 3.3.0 compatibility though: R 3.3.0 had a broken
configure script which failed to understand that zlib version
1.2.11 (installed) > 1.2.5 (required).

DESCRIPTION

@@ -1,13 +1,13 @@
 Package: vegan
 Title: Community Ecology Package
-Version: 2.5-1
-Date: 2018-04-13
+Version: 2.5-2
+Date: 2018-04-24
 Author: Jari Oksanen, F. Guillaume Blanchet, Michael Friendly, Roeland Kindt,
    Pierre Legendre, Dan McGlinn, Peter R. Minchin, R. B. O'Hara,
    Gavin L. Simpson, Peter Solymos, M. Henry H. Stevens, Eduard Szoecs,
    Helene Wagner
 Maintainer: Jari Oksanen <jari.oksanen@oulu.fi>
-Depends: permute (>= 0.9-0), lattice, R (>= 3.1.0)
+Depends: permute (>= 0.9-0), lattice, R (>= 3.3.0)
 Suggests: parallel, tcltk, knitr
 Imports: MASS, cluster, mgcv
 VignetteBuilder: utils, knitr