[bash-completion] Always backup existing completion definitions

_fzf_completion_loaded is no longer checked. This change increases the
load time by a few milliseconds, but I can't think of a better way to
handle the issue.

Close #783.

shell/completion.bash

@@ -261,12 +261,9 @@ a_cmds="
 x_cmds="kill ssh telnet unset unalias export"
 
 # Preserve existing completion
-if [ "$_fzf_completion_loaded" != '0.11.3' ]; then
-  # Really wish I could use associative array but OSX comes with bash 3.2 :(
-  eval $(complete | command grep '\-F' | command grep -v _fzf_ |
-    command grep -E " ($(echo $d_cmds $a_cmds $x_cmds | sed 's/ /|/g' | sed 's/+/\\+/g'))$" | _fzf_orig_completion_filter)
-  export _fzf_completion_loaded=0.11.3
-fi
+eval $(complete |
+  sed -E '/-F/!d; / _fzf/d; '"/ ($(echo $d_cmds $a_cmds $x_cmds | sed 's/ /|/g; s/+/\\+/g'))$/"'!d' |
+  _fzf_orig_completion_filter)
 
 if type _completion_loader > /dev/null 2>&1; then
   _fzf_completion_loader=1