Travis: Do not ignore deleted frozen files
Travis: Do not ignore deleted frozen files

The filter `--diff-filter=d' causes deleted files to be ignored from
the frozen tools check. But there is no reason for this, as deleting
a file can break compatibility with the legacy tools.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed