fix syntastic_auto_jump functionality

Previously we removed the code to automatically set the loclist and only
set it when required. This was needed because otherwise we could
interfere with other plugins that use the loclist.

Add the call to setloclist back in only if syntastic_auto_jump is set.
This will blow away the loclist every time the file is checked, but
setting syntastic_auto_jump is the users choice.

Fixes #533