Update the INSTALL file

Fixes T4917
Differential Revision: http://phabricator.kde.org/D3722

There was still a "Krusader-15.xx" text that had be corrected. Also, the command wget http://websvn.kde.org/*checkout*/trunk/l10n-kf5/XX/messages/extragear-utils/krusader.po no longer worked, it returned "ERROR 403: Forbidden", so the instructions were changed accordingly. While at it, some modifications were made aiming to make explanations clearer for the reader.