Does not support dotted keys from toml 0.5

```console
> cargo new bar
Created binary (application) `bar` package
> cd bar
> echo 'futures.version = "0.3.5"' >>Cargo.toml
> cargo add futures-test
Command failed due to unhandled error: Unable to parse Cargo.toml

Caused by: Manifest not valid TOML
Caused by: TOML parse error at line 10, column 8
   |
10 | futures.version = "0.3.5"
   |        ^
Unexpected `.`
Expected `=`
```