Use R-release on Appveyor

The R-patched download URL is messed up around release time.
r-lib · Dec 20, 2018 · 77bf30d · 77bf30d
1 parent 1485805
commit 77bf30d
Showing 1 changed file with 1 addition and 0 deletions.
diff --git a/appveyor.yml b/appveyor.yml
@@ -47,3 +47,4 @@ environment:
   global:
     NOT_CRAN: yes
     USE_RTOOLS: true
+    R_VERSION: release