Disable backtrace CI on Rust 1.50

The `object` crate doesn't currently work on it.

    $ cargo +1.50.0 check --features backtrace
    warning: unused manifest key: package.rust-version
    error: failed to parse manifest at `/home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/object-0.30.0/Cargo.toml`

    Caused by:
      feature `resolver` is required

      this Cargo does not support nightly features, but if you
      switch to nightly channel you can add
      `cargo-features = ["resolver"]` to enable this feature

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
       - run: cargo check
       - run: cargo check --no-default-features
       - run: cargo check --features backtrace
-        if: matrix.rust != '1.39.0'
+        if: matrix.rust != '1.39.0' && matrix.rust != '1.50.0'
 
   windows:
     name: Windows