don't try to unexport PROMPT_COMMAND on bash < 4.2 (GH liquidprompt#474)

Because "declare -g" is only available since bash < 4.2
http://git.savannah.gnu.org/cgit/bash.git/tree/NEWS?id=a0c0a00fc419b7bc08202a79134fcd5bc0427071#n428

liquidprompt

@@ -1118,7 +1118,10 @@ prompt_on()
         # incorrectly export it from their profile/bashrc (GitHub #450),
         # so we preventively UNexport it.
         # TODO: warn the user if it was exported
-        declare -g +x PROMPT_COMMAND
+        if (( ${BASH_VERSION%%.*} > 4 )) || [[ ${BASH_VERSION} > 4.2 ]]; then
+            # -g is only available since bash 4.2
+            declare -g +x PROMPT_COMMAND
+        fi
 
         # Disable parameter/command expansion from PS1
         shopt -u promptvars