Fix remote Travis config extraction

The remote Travis config extraction searches the rustfmt README
for the line above the Travis config. That line was modified in
rust-lang/rustfmt#4554. This commit incorporates that change into
the config extraction logic.

Author: davidalber

tests/config_validation.rs

@@ -25,7 +25,7 @@ fn get_remote_config() -> String {
     let parser = Parser::new(&remote_config)
         .skip_while(|event| match event {
             &Event::Text(ref s) => s
-                != "A minimal Travis setup could look like this (requires Rust 1.24.0 or greater):",
+                != "A minimal Travis setup could look like this (requires Rust 1.31.0 or greater):",
             _ => true,
         })
         .skip_while(|event| match event {