Update rust-toolchain to nightly-2019-04-17

[koushiro]

Signed-off-by: koushiro [email protected]

Update ustd and primitive-types.

liballoc will be stablized in rust 1.36. Amazing!

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   94.27%   94.35%   +0.07%     
==========================================
  Files          13       13              
  Lines        1276     1276              
==========================================
+ Hits         1203     1204       +1     
+ Misses         73       72       -1

Impacted Files	Coverage Δ
primitive-types/src/lib.rs	50% <ø> (ø)	⬆️
ustd/src/lib.rs	92.3% <ø> (ø)	⬆️
primitive-types/src/tests.rs	97.97% <ø> (ø)	⬆️
rlp/src/error.rs	50% <0%> (+50%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 98e7397...c3e8394. Read the comment docs.