Remove build script from Cargo.toml

I copied this from another crate and forgot to remove this line.

Author: davidpdrsn

Cargo.toml

@@ -1,7 +1,6 @@
 [package]
 version = "0.1.0" # remember to update html_root_url
 authors = ["David Pedersen <david.pdrsn@gmail.com>"]
-build = "build.rs"
 categories = ["development-tools"]
 description = "Easily compare two JSON values and get great output"
 homepage = "https://github.com/davidpdrsn/assert-json-diff"