Change DEB nodejs relation from Depends to Recommends (#916)

* Change DEB nodejs relation from Depends to Recommends

A "Recommends" relation preserves the out-of-the-box experience for most people,
as recommended packages are installed by default on modern Debian / Ubuntu
distros. But it allows people who install Node.js in a different manner to opt
out of the nodejs DEB package.

* Add "Conflicts" relationship with DEB nodejs < 4.0.0

resources/debian/control.in

@@ -1,6 +1,7 @@
 Package: yarn
 Version: $VERSION-1
-Depends: nodejs (>= 4.0.0)
+Recommends: nodejs
+Conflicts: nodejs (< 4.0.0)
 Section: devel
 Priority: optional
 Architecture: all