Widen range of recommended replacement module versions

**what is the change?:**
Instead of recommending specific versions of 'prop-types' and
'create-react-class' we ask folks to use the latest available 15.* version.

**why make this change?:**
The latest version of those plugins, within the 15 version, is the most likely
to be stable. For example, we know that 'react-create-class' is broken for AMD
builds at the current latest version, so once we release a fix then the
recommended version will change.

**test plan:**
Visual inspection
Also fixed some tests.

src/isomorphic/React.js

@@ -112,8 +112,7 @@ if (__DEV__) {
           didWarnPropTypesDeprecated,
           'Accessing PropTypes via the main React package is deprecated,' +
             ' and will be removed in  React v16.0.' +
-            ' Use the prop-types package from npm instead.' +
-            ' Version 15.5.10 provides a drop-in replacement.' +
+            ' Use the latest available 15.* prop-types package from npm instead.' +
             ' For info on usage, compatibility, migration and more, see ' +
             'https://fb.me/prop-types-docs',
         );
@@ -129,7 +128,7 @@ if (__DEV__) {
           'Accessing createClass via the main React package is deprecated,' +
             ' and will be removed in React v16.0.' +
             " Use a plain JavaScript class instead. If you're not yet " +
-            'ready to migrade, create-react-class v15.5.3 is available ' +
+            'ready to migrade, create-react-class v15.* is available ' +
             'on npm as a temporary, drop-in replacement. ' +
             'For more info see https://fb.me/react-create-class',
         );

src/isomorphic/__tests__/React-test.js

@@ -46,10 +46,10 @@ describe('React', () => {
     expect(console.warn.calls.count()).toBe(1);
     expect(console.warn.calls.argsFor(0)[0]).toContain(
       'Warning: Accessing createClass via the main React package is ' +
-        'deprecated, and will be removed in React v16.0. Use a plain ' +
-        "JavaScript class instead. If you're not yet ready to migrade," +
-        ' create-react-class v15.5.3 is available on npm as a ' +
-        'temporary, drop-in replacement. ' +
+        'deprecated, and will be removed in React v16.0. ' +
+        'Use a plain JavaScript class instead. ' +
+        "If you're not yet ready to migrade, create-react-class " +
+        'v15.* is available on npm as a temporary, drop-in replacement. ' +
         'For more info see https://fb.me/react-create-class',
     );
   });
@@ -63,8 +63,7 @@ describe('React', () => {
     expect(console.warn.calls.argsFor(0)[0]).toContain(
       'Warning: Accessing PropTypes via the main React package is ' +
         'deprecated, and will be removed in  React v16.0. ' +
-        'Use the prop-types package from npm instead. ' +
-        'Version 15.5.10 provides a drop-in replacement. ' +
+        'Use the latest available 15.* prop-types package from npm instead. ' +
         'For info on usage, compatibility, migration and more, ' +
         'see https://fb.me/prop-types-docs',
     );