Update the minimal Rust version

ACMEd can no longer compile on Rust 1.28 and lower because of the
`ucd-trie` dependency.

.travis.yml

@@ -2,9 +2,6 @@ language: rust
 dist: xenial
 
 rust:
-    - "1.36.0"
-    - "1.37.0"
-    - "1.38.0"
     - "1.39.0"
     - "1.40.0"
     - "1.41.1"

README.md

@@ -59,7 +59,7 @@ This project provides the following man pages:
 
 ## Build from source
 
-In order to compile ACMEd, you will need the [Rust](https://www.rust-lang.org/) compiler and its package manager, Cargo. The minimal required Rust version is 1.36.0, although it is recommended to use the latest stable one.
+In order to compile ACMEd, you will need the [Rust](https://www.rust-lang.org/) compiler and its package manager, Cargo. The minimal required Rust version is 1.39.0, although it is recommended to use the latest stable one.
 
 ACMEd depends OpenSSL 1.1.0 or higher.