Update minimum required Rust version to 1.40.0

This change updates the minimum required version of Rust to 1.40.0.
Among other changes, this version stabilized std::mem::take, which is
something we will use with subsequent changes.

CHANGELOG.md

@@ -1,6 +1,7 @@
 Unreleased
 ----------
 - Added bash completion support via `shell-complete` utility program
+- Updated minimum required Rust version to `1.40.0`
 
 
 0.3.2

README.md

@@ -1,6 +1,6 @@
 [![pipeline](https://gitlab.com/d-e-s-o/nitrocli/badges/master/pipeline.svg)](https://gitlab.com/d-e-s-o/nitrocli/commits/master)
 [![crates.io](https://img.shields.io/crates/v/nitrocli.svg)](https://crates.io/crates/nitrocli)
-[![rustc](https://img.shields.io/badge/rustc-1.35+-blue.svg)](https://blog.rust-lang.org/2019/05/23/Rust-1.35.0.html)
+[![rustc](https://img.shields.io/badge/rustc-1.40+-blue.svg)](https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html)
 
 nitrocli
 ========

ci/gitlab-ci.yml

@@ -4,8 +4,8 @@
 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/rust/tags/
 # The recipe for this docker image can be found at:
-# https://github.com/rust-lang/docker-rust/blob/29bf41a2cc4fb8d3f588cf51eb6a8ba883808c4b/1.35.0/stretch/Dockerfile
-image: "rust:1.35.0"
+# https://github.com/rust-lang/docker-rust/blob/3898d19194231639f1afc3096bd04702eaf555e7/1.40.0/buster/Dockerfile
+image: "rust:1.40.0"
 
 build-test:cargo:
  script: