Upgrade Semaphore CI

Author: Saluki

.semaphore/semaphore.yml

@@ -3,16 +3,18 @@ name: Rust
 agent:
   machine:
     type: e1-standard-2
-    os_image: ubuntu1804
+    os_image: ubuntu2004
   containers:
+    # Rust 1.56 (2021 edition) is currently not supported
     - name: main
-      image: 'registry.semaphoreci.com/rust:1.47'
+      image: 'registry.semaphoreci.com/rust:1.51'
 blocks:
   - name: Test release
     task:
       jobs:
         - name: Cargo test
           commands:
+            - rustup update
             - checkout
             - cargo build --verbose
             - cargo test --verbose