Add fish prompt changing (#475)

bin/pyenv-sh-deactivate

@@ -187,7 +187,15 @@ esac
 
 case "${shell}" in
 fish )
-  :
+  cat <<EOS
+# check if old prompt function exists
+if functions -q _pyenv_old_prompt
+  # remove old prompt function if exists.
+  functions -e fish_prompt
+  functions -c _pyenv_old_prompt fish_prompt
+  functions -e _pyenv_old_prompt
+end
+EOS
   ;;
 * )
   cat <<EOS