Update dependencies, min Rust version to 1.26.2

.appveyor.yml

   - TARGET: x86_64-pc-windows-msvc
     CHANNEL: stable
   - TARGET: x86_64-pc-windows-msvc
-    CHANNEL: 1.25.0
+    CHANNEL: 1.26.2
   - TARGET: x86_64-pc-windows-gnu
     CHANNEL: stable
   - TARGET: x86_64-pc-windows-gnu

.travis.yml

   - stable
   - beta
   - nightly
-  - 1.25.0
+  - 1.26.2
 cache: cargo
 dist: trusty
 sudo: false

Cargo.toml

 env_logger = "0.5.0-rc.2"
 gl = "0.10"
 glutin = "0.18"
-image = "0.19"
+image = "0.20"
 lazy_static = "1.0"
 linked-hash-map = "0.5.0"
 log = "0.4"
 num_cpus = "1.0"
 osmpbf = "0.1"
 regex = "1.0"
-reqwest = "0.8"
+reqwest = "0.9"
 scoped_threadpool = "0.1"
 toml = "0.4"
 

README.md

 --------
 DeltaMap is written in Rust, so you will need to [install
 Rust](https://www.rust-lang.org/install.html) to compile the program. The
-minimum supported version of Rust for DeltaMap is **1.25**.
+minimum supported version of Rust for DeltaMap is **1.26.2**.
 
 On Linux you will also need OpenSSL with headers.
 (see <https://docs.rs/crate/openssl/0.10.12> for details)

src/texture.rs

 use ::context;
 use ::image;
 use context::{Context, TextureUnit};
-use image::GenericImage;
+use image::GenericImageView;
 use std::os::raw::c_void;
 
 #[derive(Clone, Debug)]