run mysql_secure_installation to secure the freshly installed mysql server

Fix spelling error and make sure you never log in as git user so you never have to logout to run sudo.

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`.