fix git url (use `://` not `@`)

library/r-base

@@ -1,3 +1,3 @@
 # maintainer: "Carl Boettiger & Dirk Eddelbuettel" [email protected] (@cboettig, @eddelbuettel)
 
-latest: git@github.com/rocker-org/rocker@efc246b8d93905fe3d61e1aea58c8f685c14f3b6 debian/r-base
+latest: git://github.com/rocker-org/rocker@efc246b8d93905fe3d61e1aea58c8f685c14f3b6 debian/r-base