Removing dependency on reagent

[shen-tian]

Fixes #24

As it stands now, reagent 0.8 and, thus, React 16 is pulled in. re-frame 10.x still depends on reagent 0.7/React 15.

Technically, this might break a project that was relying on reagent 0.8 features. This primarily involves:

• Fragments: [:<>] tags
• Class names as collections: [:div {:class ["header" "header-large"]}]
• Error boundaries. Form-3 components with the :component-did-catch method

But they can then just add the explicit dependency for reagent 0.8.1 to get status quo back.

[shen-tian]

@deg : the main changes are on the view side of things, so doesn't affect this lib. And I had a look through the firestore example, and nothing there stands out.

[deg]

Sorry for my delay. Got lost in my inbox.