Add 'jdk' as a dependency.

Add jdk as a dependency so that R has support for java and allow
packages that need java to work.

var/spack/repos/builtin/packages/R/package.py

@@ -71,6 +71,7 @@ class R(Package):
     depends_on('tk')
     depends_on('curl')
     depends_on('pcre')
+    depends_on('jdk')
 
     def install(self, spec, prefix):
         rlibdir = join_path(prefix, 'rlib')