provide testthat version dependency

Author: jjallaire

DESCRIPTION

@@ -18,7 +18,7 @@ Description: High level functions for doing parallel programming with Rcpp.
   For example, the parallelFor() function can be used to convert the work of 
   a standard serial "for" loop into a parallel one and the parallelReduce() 
   function can be used for accumulating aggregate or other values.
-Suggests: Rcpp, testthat
+Suggests: Rcpp, testthat (>= 0.9.1)
 SystemRequirements: GNU make, Windows: cmd.exe and cscript.exe
 License: GPL-2
 URL: http://rcppcore.github.io/RcppParallel, https://github.com/RcppCore/RcppParallel