Update installation instructions to include fortran compiler dependency

[cboddy]

Hello,

Enoying this superb book so far.

When trying to build the python environment (on ubuntu 18.04) ala:

mkvirtualenv bayes
pip install -r requirements.txt

I received several cryptic error messages about pymc not installing to do with an unavailable numpy module. After some digging I found the cause to be that pymc requires a fortran compiler to be installed.

After installing gfortran it went smoothly.

I'd suggest adding a note somewhere in the README to inform people that gfortran or equivalent is required to build.

[CamDavidsonPilon]

Curious what system are you using?

[cboddy]

chris@vulcan: ~$ uname -a
Linux vulcan 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux