Stop using deprecated Files.toString

Replace with Files.asCharSource as recommended in the deprecation
notice.

Change-Id: Ib8491c1d63a2f1b2c5e8b5d9c28cdde220a43c49
diff --git a/src/main/java/com/criteo/gerrit/plugins/automerge/PluginComment.java b/src/main/java/com/criteo/gerrit/plugins/automerge/PluginComment.java
index 4cacdd6..0ac8aff 100644
--- a/src/main/java/com/criteo/gerrit/plugins/automerge/PluginComment.java
+++ b/src/main/java/com/criteo/gerrit/plugins/automerge/PluginComment.java
@@ -28,7 +28,7 @@
   String getContent() {
     if (templatePath.exists()) {
       try {
-        return Files.toString(templatePath, Charsets.UTF_8);
+        return Files.asCharSource(templatePath, Charsets.UTF_8).read();
       } catch (final IOException exc) {
         log.error("Not able to read " + templatePath, exc);
       }