Updated Debian/Ubuntu instructions

(These instructions should be updated for Spock and later)

On modern versions of Debian and Ubuntu (Ubuntu 14.04+, Debian 8+), the "apt" command makes this much easier; you can simply run

apt-get update
apt install ./couchbase-server.deb

and it will automatically download and install the appropriate libssl and python dependencies. (Note that the "./" in that command is necessary - you have to provide something that looks like a path, or apt will think it is the name of a package.)

On earlier versions of Debian, the equivalent commands are

apt-get update
dpkg -i ./couchbase-server.deb
apt-get -f install

There is no longer any need to manually install libssl first, so those steps in the instructions should be deleted.