bump minimum Rust version to 1.26

1.25 also fails with the upgraded reffers because u128 isn't stable:

error[E0658]: 128-bit type is unstable (see issue #35118)
   --> /home/travis/.cargo/git/checkouts/reffers-rs-0d00fc7f893338b3/49a4d75/src/rc_bitmask.rs:194:34
    |
194 | rc_bit_mask_internal!(primitive, u128, 42, 42, 42);
    |                                  ^^^^

.travis.yml

@@ -4,7 +4,7 @@ language: rust
 rust:
   - stable
   - nightly
-  - 1.25.0
+  - 1.26.0
 addons:
   apt:
     packages:

guide/install-manual.md

@@ -44,7 +44,7 @@ all non-Rust dependencies:
                    libssl-dev \
                    pkgconf
 
-Next, you need Rust 1.25+ and Cargo. The easiest way to install them is by
+Next, you need Rust 1.26+ 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="1.25"
+RUSTC_MIN_VERSION="1.26"
 FFMPEG_MIN_VERSION="55.1.101"
 FFMPEG_RELEASE_VERSION="3.4"