Minor css typo in switch-handle

[richworx]

The css class rendered for the switch-handle span has a typo in it:

class="switch-handle btn btn-lightbtn-sm"

This is due to a typo in src/bootstrap-switch-button-react.js near line 114 (in master f206c5bf5b2):

<span className={'switch-handle btn btn-light' + (this.state.size ? 'btn-' + this.state.size : '')} />

I will submit a PR.