Added in a new method for checking pdo drivers

Added in a more dynamic method for checking whether to use dblib or
sqlsrv drivers for a mssql pdo connection. This is to make local mac
devs work well with windows prod machines.
Merge pull request #1560 from DServy/master

Added a more dynamic method for checking dblib (response to ticket #770)
Merge branch 'master' into develop
Merge branch 'develop' of github.com:laravel/laravel into develop