only use brew if brew is installed

custompkg/root/etc/skel/Library/profile.d/900.homebrew-completion.sh

@@ -1,3 +1,5 @@
-if [ -f $(brew --prefix)/etc/bash_completion ]; then
-    source $(brew --prefix)/etc/bash_completion
-fi
+if which brew 2>&1 >/dev/null ; then
+    if [ -f $(brew --prefix)/etc/bash_completion ]; then
+        source $(brew --prefix)/etc/bash_completion
+    fi
+fi