.bash_profile: Enable Bash completion if /etc/bash_completion exist…

…s too

This is useful on Linux.

Ref. mathiasbynens#278.

.bash_profile

@@ -28,6 +28,8 @@ done;
 # Add tab completion for many Bash commands
 if which brew > /dev/null && [ -f "$(brew --prefix)/etc/bash_completion" ]; then
 	source "$(brew --prefix)/etc/bash_completion";
+elif [ -f /etc/bash_completion ]; then
+	source /etc/bash_completion;
 fi;
 
 # Enable tab completion for `g` by marking it as an alias for `git`