Switch to the Rust 2018 edition

This requires Rust 1.31 (from last year) to build, but is otherwise
compatible with the 2015-edition code.

examples/cli.rs

@@ -1,5 +1,5 @@
-extern crate simple_logger;
-extern crate sqlparser;
+use simple_logger;
+
 ///! A small command-line app to run the parser.
 /// Run with `cargo run --example cli`
 use std::fs;

examples/parse_select.rs

@@ -1,5 +1,3 @@
-extern crate sqlparser;
-
 use sqlparser::dialect::GenericSqlDialect;
 use sqlparser::sqlparser::*;