Dropping support for React 15/Reagent 0.7.0

[danielcompton]

This issue is a note to others that I intend to drop support for React 15/Reagent 0.7.0 in the next major release of re-frame-10x and make React 16/Reagent 0.8.0 the minimum version requirements.

We created dual support a while ago as some people wanted to use 10x with the beta of Reagent 0.8.0. Reagent 0.8.0 is out now. Maintaining the dual releases is a pain; it requires manual work when doing a release, and the git merges are quite error prone. Dropping the dual version support means that when I spend time on 10x, I can focus on improving it. There are also bugs which are trickier to fix when supporting two versions of React/Reagent, like #224 and #201.

Are there people still stuck on Reagent 0.7.0 that can't upgrade further? Reagent 0.8.0 alphas came out in July 2017 and 0.8.0 was released in April 2018.