Update requirements

Remove packages needed for building a patched procd in
OpenWrt 18.06 and earlier.

README

@@ -7,11 +7,7 @@ Requirements
 ------------
 It's recommended you use Debian or Ubuntu on the build system. The following additional packages are required on Ubuntu 18.04:
 
-* build-essential
-* subversion
 * fakeroot
-* gawk
-* gpg
 
 Configuration
 -------------