Add description and dependencies to `mod.conf`

The legacy `depends.txt` and `description.txt` files are no longer
needed since Minetest 5.0.0 or later will read everything from
`mod.conf`.

This raises the minimum supported Minetest version to 5.0.0.

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Polish translation.
 
+### Changed
+
+- The minimum supported Minetest version is now 5.0.0.
+
 ## [1.1.0] - 2019-03-23
 
 ### Added

depends.txt

-default
-farming?
-intllib?
-mg?

description.txt

-Adds new Ore types.

mod.conf

 name = moreores
+description = Adds new ore types.
+depends = default
+optional_depends = farming,intllib,mg