Allow react_component to accept JSON strings

Rails ships with a useful JSON builder that makes rendering to strings
of JSON via templates very easy, but rendering from objects to json
not quite so easy. It's used by a lot of Rails apps as the default way
to serialize objects to JSON.

This allows render_component to take a string as the props argument,
and instead of calling #to_json on it, just pass it straight through
to the renderer.

Basically, this allows syntax like the following:

```ruby
react_component(
  'Todo',
  render(template: 'todos/show.json.jbuilder', locals: {todo: @todo}),
  {prerender: true}
)
```
Merge pull request #95 from xionon/jbuilder-integration

Allow react_component to accept JSON strings
- Enable emitting warnings by default
- `File.exists?` is deprecated, changed to use `File.exist?` instead.
- Fixed react renderer and view helper to not throw ambiguous arguments warnings, when asserting data using regex arguements.
update to react-source 0.12 final
update docs and test examples for changes in 0.12
fix deprecated calls and provide expected messages for 0.12
Merge pull request #105 from grahamcracker/0.12

update to react-source 0.12 final
Suppress deprecation waring from appraisal

The following warning is suppressed.
```
`rake appraisal:install` task is deprecated and will be removed soon.
Please use `appraisal install`.
```
Test against Ruby 2.1 on Travis CI
Merge pull request #109 from tricknotes/suppress-warning

Suppress deprecation waring from appraisal
Merge pull request #110 from tricknotes/ruby-21

Test against Ruby 2.1 on Travis CI
Test against Ruby 2.2 on Travis CI
Merge pull request #112 from tricknotes/ruby-22

Test against Ruby 2.2 on Travis CI
Remove deprecated calls from README.md
Merge pull request #100 from vipulnsward/100-fix-warnings

Fix warnings
Merge pull request #114 from conceptualitis/master

Remove deprecated calls from README.md
Merge remote-tracking branch 'upstream/master' into component-generator
Changes to work with React 0.12 and follow @zpao's recommendations