Nix minimal version: 1.10 -> 1.11

`ruby` attribute requires features from 1.11
```
$ nix-build -A ruby
error: cannot coerce a set to a string, at pkgs/development/interpreters/ruby/default.nix:57:17
```

Changed files
+1 -1
lib
+1 -1
lib/minver.nix
···
# Expose the minimum required version for evaluating Nixpkgs
-
"1.10"
···
# Expose the minimum required version for evaluating Nixpkgs
+
"1.11"