Install steps - Fixes mysql package name dependency for Debian/Ubuntu

There is no package called `mysql` in Debian/Ubuntu - instead, MySQL is
split into `mysql-client` and `mysql-server`.

Also deeted duplicate package name.
Install steps - Use "Linux" instead of distribution name to detect OS
Install steps - Fixed typo for installation of rvm on Linux
Install steps - Use edx-platform gemset when fetching ruby deps
Merge pull request #37 from antoviaque/install-ubuntu-dependencies

Dependency installation fixes for Debian/Ubuntu