Allow for optional SimpleCellFormatter.value prop (#326)

[chuntley]

Description

Passing in null to SimpleCellFormatter is valid, this PR removes the isRequired flag on propTypes to remove warnings from the console when null is passed as a value.

What kind of change does this PR introduce? (check one with "x")

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Prop type warning is thrown when a null value is passed to SimpleCellFormatter

What is the new behavior?
No warning is thrown

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X] No

Other information:

Fixes: #326 using michielboekhoff's suggestion in the thread.

[coveralls]

Changes Unknown when pulling b5d52b6 on chuntley:simplecell-value into ** on adazzle:master**.

[coveralls]

Changes Unknown when pulling 0cd7ac9 on chuntley:simplecell-value into ** on adazzle:master**.

[dklanac]

For those like me who are waiting for this to be merged into master, here's a quick way to incorporate @chuntley 's PR now as a temporary monkey patch:

Just pop this into your codebase and make sure that it's called after you've loaded up ReactDataGrid.

You can verify that the warning

Warning: Failed prop type: The prop value is marked as required in SimpleCellFormatter,

No longer displays in your console.

const SimpleCellFormatter = ReactDataGrid.formatters.SimpleCellFormatter

SimpleCellFormatter.propTypes = {
  value: React.PropTypes.oneOfType(
    [
      React.PropTypes.string, 
      React.PropTypes.number,
      React.PropTypes.object,
      React.PropTypes.bool
    ]
  )
}

[chuntley]

Anything else needed to get this merged in?

[chuntley]

[amanmahajan7]

@chuntley thank you for the PR. This will be released in the next minor release