Remove use of prelude_import

[crawfxrd]

Fixes the following warning:

warning: the feature `prelude_import` is internal to the compiler or standard library
 --> src/main.rs:5:12
  |
5 | #![feature(prelude_import)]
  |            ^^^^^^^^^^^^^^
  |
  = note: using it is strongly discouraged
  = note: `#[warn(internal_features)]` on by default

Resolves: #88

[jacobgkau]

This removes the later instance of this warning (an earlier instance for its use in gop-policy still occurs).