Recover `zlib.h` in `external`

Fixes #48 .

* #48 is reproduced on travis by [this commit](https://github.com/utensil/nupic.core/commit/74807e67db6062aafb28a5b9233aa49aad960b6d), see [the travis build for it](https://travis-ci.org/utensil/nupic.core/builds/24893948)
* #48 is fixed by recovering `zlib.h` in `external` from numenta:master@46835bfdf53b4b2d, right before merging #47 which caused #48