Add support for Spark 2.1.x (while retaining Spark 2.0.x support)

[JoshRosen]

This patch builds on #206 in order to restore support for Spark 2.0.x. This means that a single binary artifact can be used with both Spark 2.0.x and 2.1.x, simplifying the builds of downstream projects which are compatible with both Spark versions.

[codecov-io]

Codecov Report

Merging #212 into branch-3.2 will increase coverage by 0.78%.

@@              Coverage Diff               @@
##           branch-3.2     #212      +/-   ##
==============================================
+ Coverage       90.62%   91.41%   +0.78%     
==============================================
  Files               5        5              
  Lines             320      326       +6     
  Branches           49       54       +5     
==============================================
+ Hits              290      298       +8     
+ Misses             30       28       -2

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[bdrillard]

A pull request for #169 would require the latest changes in the ObjectType API, which are only reflected in Spark 2.1.x. I think that would mean that a version of Spark-Avro with code for #169 merged, but without explicitly Spark 2.1.x, would break if the user tried to use the feature for datasets.

[JoshRosen]

@bdrillard, my plan was to release a cross-compatible version as 3.2.0 now and then to revisit dropping 2.0.x compatibility and bumping the version number to 4.0.0 in master. In fact, let me go ahead and merge @koertkuipers's PR into master and then fork a branch-3.2 which adds my compatibility changes.

[bdrillard]

@JoshRosen that makes sense to me. I'll prepare a PR for #169, and I suppose we can make sure it gets merged into a branch/release that ensures the correct version of Spark.

[JoshRosen]

I'll publish a new release as soon as I'm in an environment with access to my GPG signing key.