$ sudo apt-get update
$ sudo apt-get install postgresql-9.3-postgis-scripts

postgresql-9.3-postgis-scripts is installed in your system.

Make ensure the postgresql-9.3-postgis-scripts package were installed using the commands given below,

$ sudo dpkg-query -l | grep postgresql-9.3-postgis-scripts*

You will get with postgresql-9.3-postgis-scripts package name, version, architecture and description in a table

postgresql-9.3-postgis-scripts package basic information: