Disable bindgen default features (#3)

Comment added by Michael Lass <bevan@bi-co.net>:
* While in bindgen 0.61 bindgen-cli was extracted and the dependency tree was reduced a lot, we still pull in unneeded dependencies. Explicitly specify runtime as the only required feature for bindgen.
* Simple version requirements are interpreted the same as caret requirements and their use is recommended.

Cargo.toml

@@ -18,7 +18,7 @@ version = "0.1.0"
 travis-ci = { repository = "michaellass/e2p-sys", branch = "master" }
 
 [build-dependencies]
-bindgen = "^0.61.0"
+bindgen = { version = "0.61", default-features = false, features = ["runtime"] }
 metadeps = "^1.1.2"
 regex = "^1.3.9"