Fix url_base_pathname init configs

[T4rk1n]

• Set url_base_pathname to None in dash.__init__ Fix Dash's config validation prevents requests_pathname_prefix and requests_pathname_prefix from being set #364 .
• Fix circleci bad cache key (was the same key for both python 3.6 and python 2.7 causing issues).

[ned2]

Cool, didn't realise the fix was that simple. I can't comment on the config.yml changes, but that fix to url_base_pathname default looks good. 💃