Some requirements changes in README.md

As I have a fresh ubuntu laptop I noticed some details some users, particulary new ones, may run into.

Chunk #1. I'm accustomed to the gcc package including g++ too, but on Ubuntu 14.04 I had to install g++ additionally before building the driver.
Chunk #2. just a note to save gperftools users from installing libtcmalloc again unnecessarily
Chunk #3. Is duplicity required? I know it as backup tool and I see it's installed by default on my new ubuntu build, but I don't think scons has a requirement.