.version: remove final newline

When reading the `nixpkgs` version from `.version` you always have a
`\n` at the end because of the final newline.

This issue exists since b7d15ed and had to be fixed several times
according to the history of `.version`.

Furthermore @Infinisil recommended I explicitly configured
`.editorconfig` to avoid newlines in `.version`.

.editorconfig

@@ -26,3 +26,7 @@ indent_size = 4
 # Match diffs, avoid to trim trailing whitespace
 [*.{diff,patch}]
 trim_trailing_whitespace = false
+
+# https://github.com/NixOS/nixpkgs/pull/39336#discussion_r183387754
+[.version]
+insert_final_newline = false

.version

@@ -1 +1 @@
-18.09
+18.09