can not install libpq-dev in ubuntu 14.04
Problem is solved!
I solved this with remove libpq5 :
sudo apt-get remove libpq5
sudo apt-get install libpq-dev
To fix this, I had to add postgresql's repo:
sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
sudo apt-get -y update
sudo apt-get remove libpq5
sudo apt-get install libpq-dev