Fix undef var while installing using vundle

Vundle complains when installing the plugin as is on master.
As per @amadanmath comment:
g:SyntacticRegistry is defined in registry.vim and used by
syntastic.vim, but the latter was loading sooner than the 
former according to :scriptnames. Including the line
source <sfile>:h/syntastic/registry.vim into syntastic.vim
somewhere before the line 108 seems to fix it.
Fixes #543