Order use statements case-insensitive, i.e. ignore their case

The pre 1.0 version of php-cs-fixer and IDEs, e.g. the one IntelliJ IDEA uses, ordered the use statements independant of their case. The OrderedUseFixer was ignoring the case. With version 1.1 the case is not ignored resulting in the wrong sort order. This change fixes that. Please review and merge.