Bump rust-crypto to v0.2.4
Fix build with rustc master

* core::slice::Items renamd to core::slice::Iter
* from_utf8 returns Result instead of Option
* Unique type per fn item. Rust Issue #19891
Merge pull request #478 from lht/fix-build-master-62fb41c32

Fix build master rustc @62fb41c32
Add /tmp/ to .gitignore
Fix deprecation warnings (`into_string` => `to_owned`)
Fix deprecation warnings on latest nightlies
Merge pull request #480 from slapresta/fix-deprecation-warnings

Fix deprecation warnings
Merge pull request #479 from slapresta/add-tmp-to-gitignore

Add /tmp/ to .gitignore