Simple update of composer requirements to support Laravel 6.*

This is a major re-write of the v0.1 logic, though the goal is to make it backwards-compatible so it's marked as a minor release update.
However (for now at least) the time series functionality is removed. This is only supported by two data providers and require extra logic and a payed subscription (making it hard to dev/test). This might be added back later if requested. If you rely on timeSeries functionality v.0.1.* should still work.

It should be easier to add new Currency Data Providers/API's in the future from now on, with a new Provider class logic.

First pre-release after code restructure. A little practical testing and some more tweaking and bug fixing is going to be needed before stable release later on.

Added Laravel .env support for config settings.
Added unit testing for all sources and test data is local file now.

Changed composer requirements to match Laravel 5.5+, phpunit 6.0+
Changed default currency data from Yahoo to source to fixer.io (Yahoo http source unreliable).
Testing in php 7.0 and Laravel 5.5+ environment.

Added support for fixer.io