Fish bindings: replace eval with source

For some reason, fish 2.5.0 freaks out under certain circumstances if
your `config.fish` calls `eval` at any point. For equally mysterious
reasons, replacing `eval` with `source` makes things work again.

See fish-shell/fish-shell#3809 for more details.

powerline/bindings/fish/powerline-setup.fish

@@ -62,7 +62,7 @@ function powerline-setup
 				set rpromptpast
 				set columnsexpr '(_powerline_columns)'
 			end
-			eval "
+			echo "
 			function fish_prompt
 				env \$POWERLINE_COMMAND $POWERLINE_COMMAND_ARGS shell $promptside $addargs
 			end
@@ -73,7 +73,7 @@ function powerline-setup
 			function _powerline_set_columns --on-signal WINCH
 				set -g _POWERLINE_COLUMNS $columnsexpr
 			end
-			"
+			" | source
 			_powerline_set_columns
 		end
 		_powerline_set_default_mode