make Checkstyle version configurable per project

[dougxc]

This addresses #74.

A project can explicitly specify the Checkstyle version:

    "com.oracle.graal.graph.test" : {
      ...
      "checkstyleVersion" : "6.15",
      ...
    }

or inherit it along with the Checkstyle config from another project:

    "com.oracle.graal.graph" : {
      ...
      "checkstyleVersion" : "6.15",
      ...
    },
    "com.oracle.graal.graph.test" : {
      ...
      "checkstyle" : "com.oracle.graal.graph",
      ...
    }