Update lintr

- remove deprecated linters: camel_case_linter, snake_case_linter --> replaced by (currently non-active) `object_name_linter` (see #338)
- activated `absolute_path_linter`: close #339

.lintr

@@ -5,16 +5,14 @@ linters: with_defaults(
   closed_curly_linter(allow_single_line = TRUE),
   # Turn off default linters for now:
   object_name_linter = NULL,
-  camel_case_linter = NULL,
-  snake_case_linter = NULL,
   #--- NON-DEFAULT LINTERS
   # Not activated non-default linters:
   #extraction_operator_linter,
   #implicit_integer_linter,
   #todo_comment_linter,
-  #absolute_path_linter(lax = TRUE),
   # Activated non-default linters:
   T_and_F_symbol_linter,
+  absolute_path_linter(lax = TRUE),
   nonportable_path_linter(lax = TRUE),
   semicolon_terminator_linter(semicolon = c("compound", "trailing")),
   seq_linter,