Skip version aliases while rehashing

Because a version alias points to another version in rbenv, its
executables will get included in the rehash process anyway.

I hoped that this results in rehash speedup because fewer directories
get processed on a system with lots of aliases (I have 9), but the truth
is I didn't witness any speedup. It's likely that the overhead of
`rbenv-versions --skip-aliases` makes up for any performance gain that
we could have had here.

libexec/rbenv-rehash

@@ -81,9 +81,12 @@ remove_outdated_shims() {
 
 # List basenames of executables for every Ruby version
 list_executable_names() {
-  local file
-  for file in "$RBENV_ROOT"/versions/*/bin/*; do
-    echo "${file##*/}"
+  local version file
+  rbenv-versions --bare --skip-aliases | \
+  while read version; do
+    for file in "${RBENV_ROOT}/versions/${version}/bin/"*; do
+      echo "${file##*/}"
+    done
   done
 }