bashrc: added a check for bash version 4 or newer

.bashrc

@@ -1,11 +1,19 @@
 # .bashrc
-
 # Source global definitions
 if [[ -r /etc/bashrc ]]; then
   . /etc/bashrc
 fi
 
+# Bash 4 or bust
+if (( BASH_VERSINFO[0] < 4 )); then
+  echo "Please get a newer version of Bash."
+  echo "Your bashrc won't work otherwise."
+else
   # Source .bashrc.d
   for file in ~/.bashrc.d/*.bashrc; do
     . "$file"
   done
+
+  unset -v file
+fi
+