Fix tests and allow travis to build nostd

Author: sorpaas

.travis.yml

@@ -12,6 +12,7 @@ os:
 script:
   - cargo build --release --all --verbose
   - cargo test --release --all --verbose
+  - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; cargo build --no-default-features; fi
 
 matrix:
   allow_failures:

src/lib.rs

@@ -29,6 +29,8 @@ extern crate block_core;
 extern crate sha3;
 extern crate digest;
 
+#[cfg(feature = "std")]
+extern crate block;
 #[cfg(feature = "std")]
 extern crate ripemd160;
 #[cfg(feature = "std")]