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.

requirements/system/ubuntu/apt-packages.txt

@@ -30,7 +30,6 @@ libreadline6-dev
 mongodb
 nodejs
 coffeescript
-mysql
-libmysqlclient-dev
+mysql-client
 virtualenvwrapper
 libgeos-ruby1.8