Removed manual calls to React.PropTypes

* Got a warning that a manual call to React.Proptypes won't be supported in production for the next major versions.
* I just removed them for these three components and moved them inside the class definition.

imports/ui/components/Button.jsx

@@ -4,6 +4,10 @@ let Link;
 try { Link = require('react-router').Link; } catch(e) {}
 
 export class Button extends React.Component {
+  propTypes: {
+    onClick: React.PropTypes.func
+  }
+
   render () {
     const {
       label,
@@ -27,8 +31,5 @@ export class Button extends React.Component {
                    onClick={ onClick }>{ label }</button>;
   }
 }
-Button.propTypes = {
-  onClick: React.PropTypes.func
-};
 
 Accounts.ui.Button = Button;

imports/ui/components/Field.jsx

@@ -2,6 +2,10 @@ import React from 'react';
 import { Accounts } from 'meteor/accounts-base';
 
 export class Field extends React.Component {
+  propTypes: {
+    onChange: React.PropTypes.func
+  }
+
   constructor(props) {
     super(props);
     this.state = {
@@ -68,8 +72,5 @@ export class Field extends React.Component {
     ) : null;
   }
 }
-Field.propTypes = {
-  onChange: React.PropTypes.func
-};
 
 Accounts.ui.Field = Field;

imports/ui/components/PasswordOrService.jsx

@@ -4,6 +4,10 @@ import { T9n } from 'meteor/softwarerero:accounts-t9n';
 import { hasPasswordService } from '../../helpers.js';
 
 export class PasswordOrService extends React.Component {
+  propTypes: {
+    oauthServices: React.PropTypes.object
+  }
+
   constructor(props) {
     super(props);
     this.state = {
@@ -32,8 +36,5 @@ export class PasswordOrService extends React.Component {
     return null;
   }
 }
-PasswordOrService.propTypes = {
-  oauthServices: React.PropTypes.object
-};
 
 Accounts.ui.PasswordOrService = PasswordOrService;