Handle the case when there is no processor info in `/proc/cpuinfo`

`grep -c` would correctly output "0", which is handled specially later
on, but would also exit with a nonzero status which would abort the
script. Piping the output to `wc` to do the counting makes the exit
status of grep irrelevant.

Fixes #447

bin/ruby-build

@@ -101,7 +101,7 @@ num_cpu_cores() {
   if [ "Darwin" = "$(uname -s)" ]; then
     num="$(sysctl -n hw.ncpu 2>/dev/null || true)"
   elif [ -r /proc/cpuinfo ]; then
-    num="$(grep -c ^processor /proc/cpuinfo)"
+    num="$(grep ^processor /proc/cpuinfo | wc -l)"
     [ "$num" -gt 0 ] || num=""
   fi
   echo "${num:-2}"