update license

Author: kevinushey

DESCRIPTION

@@ -25,7 +25,7 @@ Suggests:
     knitr,
     rmarkdown
 SystemRequirements: GNU make, Windows: cmd.exe and cscript.exe, Solaris: g++ is required
-License: GPL-2
+License: GPL (>= 2)
 URL: http://rcppcore.github.io/RcppParallel, https://github.com/RcppCore/RcppParallel
 BugReports: https://github.com/RcppCore/RcppParallel/issues
 Biarch: TRUE