FIX: Missing argument 2 in is_fixture_exists()

I copied v.1.4.7 into my own branch, merged some changes into that branch and am submitting this pull request. Not sure why it's showing so many files as being changed, but I did a folder diff from my branch and your v.1.4.7 and there are only 3 file diffs
-readme.txt
-phpleague.php
-libs/phpleague-database.php

I will be using this branch for all future pull requests