add missing dependencies to Package-Requires

The `Package-Requires` header should (presumably) reflect packages which are required by the body, but was missing entries for `anaphora` and `combinators`.

I'm no elisp packaging expert, but when I loaded this package from marmalade via el-get, el-get thought it only depended on `macro-utils`, and it looks like this was due to the incomplete `Package-Requires` header.