Add GDAL version environment variable support

[hectcastro]

Allow library consumers to define the version of GDAL Java bindings installed in the environment geotrellis-gdal intends to be executed in. Also, add the beginnings of a compatibility matrix for geotrellis-gdal and GDAL version pairings.

Fixes https://github.com/geotrellis/geotrellis-gdal/issues/37

---

Testing

$ docker run --rm \
  -v $(pwd)/build.sbt:/root/build.sbt \
  -v $(pwd)/project:/root/project \
  -v $(pwd)/gdal:/root/gdal \
  -v $(pwd)/src:/root/src \
  -v $(pwd)/sbt:/root/sbt \
  -w /root \
  -e GDAL_VERSION=2.3.2 \
  quay.io/azavea/openjdk-gdal:2.3.2-slim \
  ./sbt "project gdal" test

...

[info] Run completed in 26 seconds, 109 milliseconds.
[info] Total number of tests run: 47
[info] Suites: completed 3, aborted 0
[info] Tests: succeeded 47, failed 0, canceled 0, ignored 3, pending 0
[info] All tests passed.
[success] Total time: 77 s, completed Jan 10, 2019 3:07:22 AM

[pomadchin]

Nice, thank you!