Auto merge of #294 - jdm:promisefeature, r=nox

Add feature to enable native promises.

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Author: bors-servo

Cargo.toml

@@ -22,6 +22,7 @@ doctest = false
 
 [features]
 debugmozjs = ['mozjs_sys/debugmozjs']
+promises = ['mozjs_sys/promises']
 
 [dependencies.mozjs_sys]
 git = "https://github.com/servo/mozjs"