nebula: use codeload instead of github source code

Previous code was downloading file v1.3.0, which is wrong, because in
the dl folder there might be some tarballs with that naming and they are
wrong as well.

This could lead to some issues like this:
Hash of the local file v1.3.0.tar.gz does not match (file: 87cf846b02dde6328b84832287d8725d91f12f41366eecb4d59eeda1d6c7efdf, requested: b94fba0251a4a436e25b127d0b9bc0181b991631f1dc8e344b1c8e895b55375d) - deleting download.

Even though, if you tried it on SDK or minimal build when there is a
small number of packages, you most likely don't encounter it.

The correct solution is to download files with their name and version.
E.g. nebula-version.tar.gz as it is in PKG_SOURCE variable now.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

net/nebula/Makefile

@@ -8,8 +8,8 @@ PKG_VERSION:=1.3.0
 PKG_RELEASE:=1
 PKG_MAINTAINER:=Stan Grishin <stangri@melmac.net>
 
-PKG_SOURCE:=v$(PKG_VERSION).tar.gz
-PKG_SOURCE_URL:=https://github.com/slackhq/nebula/archive/
+PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
+PKG_SOURCE_URL:=https://codeload.github.com/slackhq/nebula/tar.gz/v$(PKG_VERSION)?
 PKG_HASH:=b94fba0251a4a436e25b127d0b9bc0181b991631f1dc8e344b1c8e895b55375d
 PKG_LICENSE:=MIT
 PKG_LICENSE_FILES:=LICENSE