Building without default-features while enabling the futures feature fails

[shepmaster]

We assume that there will be String, among other things:

error[E0432]: unresolved import `crate::FromString`
 --> /Users/shep/.cargo/registry/src/index.crates.io-6f17d22bba15001f/snafu-0.7.4/src/futures/try_stream.rs:5:33
  |
5 | use crate::{Error, ErrorCompat, FromString, IntoError};
  |                                 ^^^^^^^^^^ no `FromString` in the root

error[E0412]: cannot find type `String` in this scope
   --> /Users/shep/.cargo/registry/src/index.crates.io-6f17d22bba15001f/snafu-0.7.4/src/futures/try_future.rs:123:17
    |
123 |         S: Into<String>,
    |                 ^^^^^^ not found in this scope
    |