Update README.md

Current command to clone under "Running with an existing PostgreSQL Installation" is `git clone git@github.com:ripple/ripple-rest.git` which requires users to sign in. The rest of the ReadMe uses `git clone https://github.com/ripple/ripple-rest.git` . Changing to git clone `https://github.com/ripple/ripple-rest.git` keeps the doc more consistent and allows easier access.