Modify php api version and minimum supported php version.

php/ext/google/protobuf/package.xml

@@ -10,11 +10,11 @@
   <email>protobuf-opensource@google.com</email>
   <active>yes</active>
  </lead>
- <date>2016-09-02</date>
+ <date>2016-09-23</date>
  <time>16:06:07</time>
  <version>
   <release>3.1.0a1</release>
-  <api>3.1.0</api>
+  <api>3.1.0a1</api>
  </version>
  <stability>
   <release>alpha</release>
@@ -57,14 +57,15 @@ First alpha release.
  <changelog>
   <release>
    <version>
-    <release>3.1.0</release>
-    <api>3.1.0</api>
+    <release>3.1.0a1</release>
+    <api>3.1.0a1</api>
    </version>
    <stability>
     <release>alpha</release>
     <api>alpha</api>
    </stability>
-   <date>2016-09-02</date>
+   <date>2016-09-23</date>
+   <time>16:06:07</time>
    <license uri="http://www.opensource.org/licenses/bsd-license.php">New BSD License</license>
    <notes>
 First alpha release