Minor fixes for nightly 2018-08-18 (#600)

* Don't use --export-table anymore

Because it was turned on by default in the recent nightlies.

See https://github.com/rust-lang/rust/pull/53237

* use_extern_macros stabilization

With recent nightlies rustc produces a warning

```
the feature `use_extern_macros` has been stable since 1.30.0 and no longer requires an attribute to enable
```