TLDR: revert https://mongodbcr.appspot.com/188980001/ once travis tests run on mongo 3.6+.

The $lookup with complex-value example (example 4) doesn't run on mongo versions prior to 3.6 The example test bails early if the version isn't at least 3.6. Once travis/CI is using 3.6+, we can/should remove the version check in this example.