Fix bug where styles have their media queries removed on IE

In IE 9, appending two styles with media queries to a `StyleSheet`'s
`cssText` property results in the first style's media query being
removed and that first style will always apply without the query.

For example:

```
var styleTag = document.createElement('style');
document.body.appendChild(styleTag);
styleTag.styleSheet.cssText +=
  '@media (min-width: 500px) { body { background-color: blue; } }';
styleTag.styleSheet.cssText +=
  '@media (min-width: 700px) { body { background-color: red; } }';
```

Causes the body element to be red for window widths ≥ 700px, but it is
blue otherwise, even at widths < 500px.

This change uses the `innerText` property of the `styleTag` element
instead, which avoids this issue.

src/inject.js

@@ -47,7 +47,7 @@ const injectStyleTag = (cssContents /* : string */) => {
 
     if (styleTag.styleSheet) {
         // $FlowFixMe: legacy Internet Explorer compatibility
-        styleTag.styleSheet.cssText += cssContents;
+        styleTag.innerText += cssContents;
     } else {
         styleTag.appendChild(document.createTextNode(cssContents));
     }

tests/inject_test.js

@@ -102,7 +102,7 @@ describe('injection', () => {
         });
 
         // browser-specific tests
-        it('adds to the .styleSheet.cssText if available', done => {
+        it('adds to the styleTag via innerText if styleSheet is available', done => {
             const styleTag = global.document.createElement("style");
             styleTag.setAttribute("data-aphrodite", "");
             document.head.appendChild(styleTag);
@@ -111,8 +111,8 @@ describe('injection', () => {
             injectStyleOnce("x", ".x", [{ color: "red" }], false);
 
             asap(() => {
-                assert.include(styleTag.styleSheet.cssText, ".x{");
-                assert.include(styleTag.styleSheet.cssText, "color:red");
+                assert.include(styleTag.innerText, ".x{");
+                assert.include(styleTag.innerText, "color:red");
                 done();
             });
         });