Fix Cargo features / dependencies

[dhardy]

Fixes #674 and #675

I'm looking at improving the WASM integration testing, but that can be a separate PR.

[tomaka]

Shouldn't one of the two features be enabled by default?
Right now builds will still break, as people would have to add a manual feature to their Cargo.toml.

[dhardy]

Both are.

Oh, you're talking about WASM? That's another discussion, and no change since 0.6 (or even 0.5?). A reasonable idea but a new topic — I'm also trying to improve the tests for WASM.

[tomaka]

What I mean is that if you create a dummy project, add rand = "0.6" to your Cargo.toml, and try to compile for WASM, you will still get the compile_error! that this PR fixes.
In other words, it is still breaking compared to rand v0.6.1.