Update CI to Swift 5.2 requirement

Author: gwynne

.github/workflows/test.yml

@@ -4,14 +4,14 @@ on:
 jobs:
   xenial:
     container: 
-      image: vapor/swift:5.1-xenial
+      image: vapor/swift:5.2-xenial
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
     - run: swift test --enable-test-discovery --sanitize=thread
   bionic:
     container: 
-      image: vapor/swift:5.1-bionic
+      image: vapor/swift:5.2-bionic
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1