Added Swift cache

.travis.yml

@@ -32,3 +32,4 @@ cache:
         - $HOME/.cache/pip
         - $HOME/.cache/pre-commit
         - $HOME/.rustup
+        - $HOME/.swift

testing/get-swift.sh

@@ -11,6 +11,6 @@ fi
 
 mkdir -p /tmp/swift
 pushd /tmp/swift
-    wget "$SWIFT_URL" -O swift.tar.gz
-    tar -xf swift.tar.gz --strip 1
+    wget -N -c "$SWIFT_URL" -O "$HOME"/.swift/swift.tar.gz
+    tar -xf "$HOME"/.swift/swift.tar.gz --strip 1
 popd