Update Magento.gitignore

The line for `install.php` blocks adding *any* file with that name. In this instance, it caused Git to ignore a file for the Aitoc_Aitsys module located at: `app/code/local/Aitoc/Aitsys/Model/Module/Install.php`
Merge pull request #688 from leek/patch-1

Update Magento.gitignore to tighten the pattern for install.php