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

.travis.yml

@@ -4,7 +4,7 @@ language: rust
 rust:
   - stable
   - nightly
-  - 1.21.0
+  - 1.25.0
 addons:
   apt:
     packages:

guide/install-manual.md

@@ -44,7 +44,7 @@ all non-Rust dependencies:
                    libssl-dev \
                    pkgconf
 
-Next, you need Rust 1.21+ and Cargo. The easiest way to install them is by
+Next, you need Rust 1.25+ and Cargo. The easiest way to install them is by
 following the instructions at [rustup.rs](https://www.rustup.rs/).
 
 Finally, building the UI requires [yarn](https://yarnpkg.com/en/).

scripts/script-functions.sh

@@ -40,7 +40,7 @@ fi
 NODE_MIN_VERSION="8"
 YARN_MIN_VERSION="1.0"
 CARGO_MIN_VERSION="0.2"
-RUSTC_MIN_VERSION="1.21"
+RUSTC_MIN_VERSION="1.25"
 FFMPEG_MIN_VERSION="55.1.101"
 FFMPEG_RELEASE_VERSION="3.4"