Replaced gksu with lxqt-sudo for debian package making - (Solves - #40)

[pranav1698]

Short description

This replaces gksu with lxqt-sudo for the making of the systemlock package
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Fixes #40

[pranav1698]

Also I have updated the travis.yml for the proper installation and building of the systemlock debian package

[tabesin]

The travis build process environment neither lxqt-sudo nor gksu as it runs headless.

[pranav1698]

I think but this is required either-wise the systemlock debian package will not be build during travis because of unmet build dependencies
Refer this log: https://travis-ci.org/pranav1698/meilix-systemlock/builds/474873706

[pranav1698]

@abishekvashok @tabesin @meets2tarun please review

[pranav1698]

@tabesin please review