Don't use deprecated Files.toString()

Replace with Files.asCharSource(...).read() as recommended in the
deprecation notice:

https://google.github.io/guava/releases/22.0/api/docs/com/google/common/io/Files.html#toString-java.io.File-java.nio.charset.Charset-

Change-Id: I4df18b1e1b8864cc2b3f4ac86c1d2da29d6de1b0

src/main/java/com/criteo/gerrit/plugins/automerge/ReviewUpdater.java

diff --git a/src/main/java/com/criteo/gerrit/plugins/automerge/ReviewUpdater.java b/src/main/java/com/criteo/gerrit/plugins/automerge/ReviewUpdater.java
index 7a23693..1243932 100644
--- a/src/main/java/com/criteo/gerrit/plugins/automerge/ReviewUpdater.java
+++ b/src/main/java/com/criteo/gerrit/plugins/automerge/ReviewUpdater.java
@@ -43,7 +43,8 @@
 
   private String getCommentFromFile(final String filename) {
     try {
-      return Files.toString(new File(config.getTemplatesPath(), filename), Charsets.UTF_8);
+      return Files.asCharSource(new File(config.getTemplatesPath(), filename), Charsets.UTF_8)
+          .read();
     } catch (final IOException exc) {
       final String errmsg =
           String.format(