Add TODO about core2 and the test feature matrix

`core2` is not part of the feature matrix; the core2 feature cannot be
built with Rust 1.29

```
error: unable to get packages from source

Caused by:
failed to parse manifest at `/home/tobin/.cargo/registry/src/github.com-1ecc6299db9ec823/core2-0.3.3/Cargo.toml`

Caused by:
editions are unstable

Caused by:
feature `edition` is required
```

Add a TODO to `test.sh` to remind us to add `core2` to the features
matrix once we bump MSRV.

contrib/test.sh

@@ -1,5 +1,6 @@
 #!/bin/sh -ex
 
+# TODO: Add core2 here once we bump MSRV.
 FEATURES="serde serde-std std"
 
 if [ "$DO_ALLOC_TESTS" = true ]; then