FindEigen3.cmake sets EIGEN3_INCLUDE_DIR not EIGEN3_INCLUDE_DIRs.
This make CMakeLists.txt reference the correct variable.
Also include the parent path `/eigen3/...` instead of the path
found by FindEigen3.cmake so the `#include <eigen3/Eigen/*>` statements
work.

So cmake can find libraries (e.g. eigen3) installed with conda.

Fixes inclusion of Eigen3

Fixes for Ubuntu 1604

The Travis build 60.2 complains

    #error This file was generated by an older version of protoc which is

    In file included from
    /home/travis/build/posterior/distributions/src/io/schema.pb.cc:4:0:
    #error This file was generated by an older version of protoc which is
    incompatible with your Protocol Buffer headers.

- Adds protobuf as a dependency of `prepare_cc` in Makefile.

- Adds pyton-pil as a dependency, since python-scipy requires it for
  scipy.misc.imread (used in examples/mixture/main.py:48)

    https://docs.scipy.org/doc/scipy/reference/misc.html

- Adds protobuf-compiler as an apt dependency in .travis.yml.

- Adds python-pyflakes as an apt dependency in .travis.yml,
  required for `make protobuf`.

- Removes ppa:cython-dev/master-ppa repository, since cython>=0.20.1 is
  available in Ubuntu 14.04 (which the target platform of the Travis build).

- Adds pyflakes as a pip dependency before calling make, since `make protobuf`
  requires pyflakes. Note that Ubuntu 14.04 does not have python-pyflakes in apt,
  whereas Ubuntu 16.04 has python-pyflakes.

Not sure how this change is going to impact the installation from pip.

20171106 fsaad travis ubuntu1404

Fix numpy strict conversion error, (related to issue #15).

Update distributions to Python 3.

A few more Python 2->3 fixes.