gumdrop_derive does not compile with minimal versions

[Alan Somers]

The crate versions specified in gumdrop_drive's Cargo.toml file are too low. It can't compile when using those versions. To demonstrate, do

$ cargo update -Zminimal-versions
$ cargo check

on any project that uses gumpdrop. It will fail with errors like this:

error[E0599]: no method named `get_ident` found for reference `&syn::Path` in the current scope
    --> /home/somers/.cargo/registry/src/github.com-1ecc6299db9ec823/gumdrop_derive-0.8.0/src/lib.rs:1039:52
     |
1039 |                     Meta::Path(path) => match path.get_ident() {
     |                                                    ^^^^^^^^^ help: there is an associated function with a similar name: `is_ident`