* add deadsnakes ppa to install python3.6 on Ubuntu Xenial
* updated travis distro to Ubuntu 18.04 bionic
* Revert "updated travis distro to Ubuntu 18.04 bionic"
This reverts commit 323264a924e8554da49c27a374e9a6278c5a659e.
* restrict numpy version to be less than 1.19.0
* use ubuntu packaged numpy as baseline version to test against
* downgrade minimum required version of numpy to version supported by Ubuntu Xenial
* undo explicit pip install
Cython 0.24.1 has problems with instantiating template types. FastVector<Key> in instantiated template classes (e.g. PriorFactorPose2) is understood as FastVector<size_t>, hence can't be casted to KeyVector. Moving to 0.25 solves the problem.