Cannot build on rust 1.46

[rtyler]

This is an error I saw on a Linux/amd64 machine while building the rust bindings. My guess is we just need to document the right version of the compiler and toolchain we should be relying upon

➜  rust git:(40443d6) rustc --version
rustc 1.46.0 (04488afe3 2020-08-24)
➜  rust git:(40443d6) cargo test --features azure               
   Compiling deltalake v0.2.0 (/home/tyler/source/github/delta-io/delta.rs/rust)
error: no rules expected the token `,`
  --> rust/tests/read_error_test.rs:12:46
   |
12 |         deltalake::DeltaTableError::NotATable,
   |                                              ^ no rules expected this token in macro call

error: aborting due to previous error

error: could not compile `deltalake`.

To learn more, run the command again with --verbose.

[rtyler]

That compiler is old enough to where I don't know if there's anything worth documenting.