GraalVM docs to use the installer from master branch (#1169)

So that we needn't update this document if there are any changes in the script.

Author: ScalaWilliam

src/sphinx/formats/graalvm-native-image.rst

@@ -21,7 +21,7 @@ To do so, run the following. It will install GraalVM 1.0.0-rc8.
 
 .. code-block:: bash
 
-  source <(curl -o - https://raw.githubusercontent.com/sbt/sbt-native-packager/6e1ee230350ce86c37b39c75f35718ac4a7f0a26/.travis/download-graalvm)
+  source <(curl -o - https://raw.githubusercontent.com/sbt/sbt-native-packager/master/.travis/download-graalvm)
 
 Build
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