Upgrade guava to 26.0-jre

To match the version used by Gerrit since 2.16.

Change-Id: I2992a1a05ef1b8defbab15afbcf4b69d571d5215
diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 4c0a4ed..51389e6 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -78,8 +78,8 @@
 
     maven_jar(
         name = "guava",
-        artifact = "com.google.guava:guava:25.1-jre",
-        sha1 = "6c57e4b22b44e89e548b5c9f70f0c45fe10fb0b4",
+        artifact = "com.google.guava:guava:26.0-jre",
+        sha1 = "6a806eff209f36f635f943e16d97491f00f6bfab",
     )
 
     GUICE_VERS = "4.2.0"