Make dpkg executable when apt is enabled

[joshuata]

Currently if you run heavyscript enable --apt it does not enable dpkg, which leads to errors like the following when you attempt to install packages

debconf: delaying package configuration, since apt-utils is not installed
Could not exec dpkg!
E: Sub-process /usr/bin/dpkg returned an error code (100)

This can be resolved by using chmod to make /usr/bin/dpkg executable