Scott Lamb 6ab416caed bump minimum Rust version
travis-ci pointed out that building with 1.21 broke with a recent dep
upgrade (8c52c36). reffers now uses nested groups of imports, which is a
feature introduced with Rust 1.25. Prior to 1.25, it fails as follows:

error: expected one of `,` or `as`, found `::`
 --> /home/travis/.cargo/git/checkouts/reffers-rs-0d00fc7f893338b3/49a4d75/src/arc.rs:6:46
  |
6 | use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
  |                                              ^^ expected one of `,` or `as` here
2018-08-25 06:33:50 -07:00
..
2018-08-25 06:33:50 -07:00
2018-08-24 21:04:13 -07:00