Add PSR-3 log implementation to phan configuration

Summary:
This fixes the following issue detected by phan:
```
app/Exceptions/Handler.php:49
PhanUndeclaredClassMethod
Call to method error from undeclared class \Psr\Log\LoggerInterface
```

Test Plan: `phan | grep Psr`

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D779

Author: dereckson

.phan/config.php

@@ -35,6 +35,7 @@
         'vendor/laravel/framework/src/Illuminate/View',
         'vendor/mockery/mockery/library',
         'vendor/netresearch/jsonmapper/src',
+        'vendor/psr/log/Psr/Log',
         'vendor/sentry/sentry/lib/Raven',
         'vendor/symfony/console',
         'vendor/symfony/http-kernel',