I recently made a script for that. Works in Debian and other Debian-based distros (Ubuntu 18.04 tested - working). The script installs libssl1.0.0 (dependency), fakeroot & gdebi for the installation and finally Viber itself.
The problem here is indeed the libcurl package (Viber requires libcurl3 but the OS already uses libcurl4, so there's your conflict).
We'll need to create 2 files (.sh) and make them executable.
touch viber.sh repackager.sh
sudo chmod +x viber.sh repackager.sh
Double click viber.sh and inside it paste this:
sudo apt-get install gdebi fakeroot
wget -O libssl1.0.0 security-cdn.debian.org/debian-security/pool/updates/main/o/openssl/libssl1.0.0_1.0.1t-1+deb8u10_amd64.deb
sudo gdebi libssl1.0.0.deb
wget -O viber.deb http://download.cdn.viber.com/cdn/desktop/Linux/viber.deb
sudo gdebi viber4debian.deb
Now, double click repackager.sh (it's important to name it "repackager.sh", otherwise it won't work) and paste this:
fakeroot sh -c "
dpkg-deb -R viber.deb tmp
echo 'Package: viber
Maintainer: Viber Media Inc <firstname.lastname@example.org>
Replaces: viber (<< 22.214.171.1245)
Depends: libqt5gui5, libxss1, libssl1.0.0, libpulse0, libasound2, libnss3, libxcomposite1, libxcursor1, libxdamage1, libcurl3, libgstreamer-plugins-base1.0-0, libgstreamer1.0-0, gstreamer1.0-plugins-base, gstreamer1.0-plugins-good, gstreamer1.0-plugins-ugly, gstreamer1.0-pulseaudio, gstreamer1.0-libav
Conflicts: viber (<< 126.96.36.1995)
Description: Free Text & Calls.
Join Viber for Desktop to for a one-of-a-kind calling and messaging experience right on your computer!
With Viber for Desktop you can make free international calls from your computer to any other Viber user, resting assured your calls and messages are protected by end-to-end encryption.
Feel free to call, text, send photos and video messages, play YouTube videos and express yourself with stickers and GIFs no matter your location.
Viber for Desktop also provides you with Viber Out’s low-rate calling plans to any landline or mobile so you can reach out to friends and family who don’t have an internet connection or cell phone.
Easily sync between your mobile’s chat history and contact list in order to start enjoying Viber for Desktop. .
' > tmp/DEBIAN/control
dpkg-deb -b tmp viber4debian.deb
Let's run the script now (requires root privilages):