Upgrade to rand 0.7.0

[fubuloubu]

Using this package alongside a project that uses the latest rand crate, and getting an incompatibility:

   Compiling pbkdf2 v0.3.0
error[E0432]: unresolved import `rand::OsRng`
 --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/pbkdf2-0.3.0/src/simple.rs:7:12
  |
7 | use rand::{OsRng, RngCore};
  |            ^^^^^ no `OsRng` in the root

Solution: just upgrade the imports

[breard-r]

The rand dependency is required only for the include_simple feature. If you do not use the simple function then, in your project, you can specify default-features = false when including pbkdf2.