Check GEM_HOME to compare recency

src/_gem

@@ -509,8 +509,18 @@ _gem_which() {
 }
 
 _gem_extensions_caching_policy() {
+  if [[ -n $GEM_HOME ]]; then
+    gem_home=$GEM_HOME
+  else
+    gem_home=$(_call_program commands ${words[1]} env gemdir)
+  fi
+
+  # Rebuild if gems directory is more recent than cache.
+  [[ $gem_home/gems -nt "$1" ]] && return 0
+
+  # Rebuild if cache is older than one week.  
   local -a oldp
-  oldp=( "$1"(Nm+1) )
+  oldp=( "$1"(Nmw+1) )
   (( $#oldp ))
 }