Rethinking 2 - Chapter 5 Edits (#198)

* removed .from_pymc3 method calls

* fixing 5.19

constrained priors in order to allow sampler to run

* remove theano dependancy

* fixed code block 5.19

* adding environment_v4.yml

createing yml file for stat-rethink2-pymc_v4 environment

* re-ran black formatting command

* fixing gitignore

* fixing gitignore

* freezing pymc package versions

It is my understanding that pymc v4 no longer has a dependency on theano, but I did not want to remove that without confirming. I have frozen the package version for theano-pymc but can remove if that is desired

* updating Chp_05 after running isort