Install gettext and subversion

On Ubuntu and Debian `gettext` library and `subversion` VCS are not installed by `build-essential` packages but on CentOS they are installed with `Development Tools`.
Since, `gettext` packages is mostly used for localization or internationalization and subversion is one of the VCS. Believe me this both server packages are used by mostly WordPress developers frequently so package those in PuPHPet box through this shell changes in `initial-setup.sh`.