Remove prompt $ from code line for uniformity

GitLab is not yet downloaded at this point in the installation guide.

* build-essential requires make, gcc
* remove libmysql++-dev: the mysql2 gem only requires libmysqlclient-dev
* remove postgresql-server-dev-9.1: the pg gem only requires libpq-dev
* replace python-dev, python-pip with python2.7: the pygments.rb gem only requires python2.7

Update docs for PostgreSQL installation

The config skeleton for postgres is called `database.yml.postgresql`,
not `database.yml.postgres`.