Ignore compatibility check failure in internal interfaces.

asyncClose() was added to com.google.cloud.spanner.Session and
asyncDeleteSession() was added to
com.google.cloud.spanner.spi.v1.SpannerRpc in #24 which resulted in
binary compatibility test failures. This config allows us to ignore the
failure.

google-cloud-spanner/clirr-ignored-differences.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- see http://www.mojohaus.org/clirr-maven-plugin/examples/ignored-differences.html -->
+<differences>
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/cloud/spanner/Session</className>
+    <method>* asyncClose()</method>
+  </difference>
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/cloud/spanner/spi/v1/SpannerRpc</className>
+    <method>* asyncDeleteSession(*)</method>
+  </difference>
+</differences>