Add support for std/no_std num-traits.

Cargo.toml

@@ -22,13 +22,10 @@ maintenance = { status = "actively-developed" }
 [features]
 default = ["std", "impl-cgmath"]
 serialization = ["serde", "serde_derive"]
-std = []
+std = ["num-traits/std"]
 impl-cgmath = ["cgmath"]
 impl-nalgebra = ["nalgebra"]
 
-[dependencies]
-num-traits = "0.2"
-
 [dependencies.nalgebra]
 version = ">=0.14, <0.17"
 optional = true
@@ -37,6 +34,10 @@ optional = true
 version = "0.16"
 optional = true
 
+[dependencies.num-traits]
+num-traits = "0.2"
+default-features = false
+
 [dependencies.serde]
 version = "1"
 optional = true