Build improvements

[clbarnes]

• Build against python 3.6, 3.7
• Drop support for python 2.6. It's been EOL for half a decade. I think it's time.
• Use pip instead of conda on travis (I know I'm to blame for using conda in the first place, but I've grown...). Conda's dependency tree resolution is so slow these days that it may have wiped out the speed gains anyway.
• Use requirements.txt to set up a full development environment
• Use extras_require to allow users to install all extra features
• (minor) add __version_info__
• Test against jsonpickle 0.9.6 and 1.0

[clbarnes]

Branch here https://github.com/clbarnes/libNeuroML/tree/79-build-improvements