Crate authors like to put dev-dependencies in dependencies due to
https://github.com/rust-lang/cargo/issues/1596

Then due to lack of maintenance this pollutes the dependency set with old
crates that no longer compile.

Here, quickcheck 0.4 eventually depends on simd which no longer compiles.

--- a/Cargo.toml
+++ b/Cargo.toml
@@ -76,10 +76,6 @@
 version = ">=0.3.0, <0.5.0"
 optional = true
 
-[dependencies.quickcheck]
-version = "0.4"
-optional = true
-
 [dependencies.r2d2]
 version = ">= 0.8, < 0.9"
 optional = true