Update comms/gammu to 1.38.4

Changes:
20170618 - 1.38.4
[-] * Improved support for Huawei E3531 and E1756.
[-] * Fixed several issues with using library on Windows.

20170523 - 1.38.3
[-] * Improved support for ZTE MF626.
[-] * Fixed USSD handling with longer codes.
[-] * Increased default value for StatusFrequency.
[-] * Improved SMSD response on signals.
[-] * Improved SMSD throughput on big queue.
[-] * Improved SMSD compatibility with Microsoft SQL Server.

(leot)