Move methods package to Depends

The 'methods' package being in Imports (and not Depends) has caused an issue for users running Rscript. For example:
  https://groups.google.com/d/msg/shiny-discuss/Gbyh1cLI1Fo/AFmZrb4NPccJ

Apparently it should go into Depends, according to John Chambers himself:
  http://r.789695.n4.nabble.com/advise-on-Depends-td4678930.html