Update minimum Rust requirement to 1.37, fixing CI build

.gitlab-ci.yml

@@ -69,7 +69,7 @@ check-nightly:
 check-old:
   <<: *check-base
   variables:
-    RUST_VERSION: "1.34.0"
+    RUST_VERSION: "1.37.0"
 
 # Build using Rust stable
 build-x86_64-linux-gnu:

README.md

@@ -421,7 +421,7 @@ before proceeding:
 ### Build requirements
 - Regular [requirements](#requirements)
 - [`git`][git]
-- [`rust`][rust] `v1.34` or higher (install using [`rustup`][rustup])
+- [`rust`][rust] `v1.37` or higher (install using [`rustup`][rustup])
 - [OpenSSL][openssl] or [LibreSSL][libressl] libraries and headers must be available
   - Linux:
     - Ubuntu, Debian and derivatives: `apt install build-essential cmake pkg-config libssl-dev`