ubuntu 14.04 skypeforlinux-64-insider.deb

skypeforlinux : 依存: libstdc++6 (>= 4.9) しかし、4.8.4-2ubuntu1~14.04.3

https://askubuntu.com/questions/726539/sudo-apt-get-install-libstdc

sudo apt-get update && sudo apt-get upgrade
sudo apt-get install build-essential
sudo apt-get install aptitude

and then try

sudo apt-get install libstdc++6

https://askubuntu.com/questions/428198/getting-installing-gcc-g-4-9-on-ubuntu

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install g++-4.9