Update Flink Runner doc

[aljoscha]

This updates to a more recent Flink version and also adds a note about matching Scala versions.

R: @JingsongLi

[asfgit]

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[JingsongLi]

LGTM.

[cphbrt]

@aljoscha The clarification about the Scala version is good.

Note that the word-count-beam example acquired from the Quickstart - Java guide currently produces the following result from mvn dependency:tree :
[INFO] +- org.apache.flink:flink-streaming-java_2.10:jar:1.2.1:runtime
So it might be premature to update the Flink version to 1.3.0 as done in this PR. (Unless the example acquired from the Quickstart guide is expecting an update soon). Of course, an update from 1.1.2 to 1.2.1 would be appropriate.

[aljoscha]

@ChristophHebert You're right that it's not the current version, it will be the Flink version that the next Beam release (2.1.0) will use. I'll change it to use the version that we currently have.

@JingsongLi thanks for reviewing!