Update toolchain version

Author: kateinoigakukun

Diff for: .github/workflows/test.yml

@@ -19,4 +19,4 @@ jobs:
           make bootstrap
           make test
         env:
-          TOOLCHAIN_DOWNLOAD: https://github.com/kateinoigakukun/swiftwasm/releases/download/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-03-05-a/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-03-05-a-osx.tar.gz
+          TOOLCHAIN_DOWNLOAD: https://github.com/swiftwasm/swift/releases/download/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-03-08-a/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-03-08-a-osx.tar.gz

Diff for: .swift-version

@@ -1 +1 @@
-wasm-DEVELOPMENT-SNAPSHOT-2020-03-06-a
+wasm-DEVELOPMENT-SNAPSHOT-2020-03-08-a