Bump Bazel version to 0.27.0

GerritForge CI is now using Bazelisk, so that we can already gather
experience with using bleeding edge Bazel version on master.

Change-Id: I9ec7228a990c9b8e4b97d1909135056668c7d4b4
diff --git a/.bazelversion b/.bazelversion
index bb73342..1b58cc1 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-0.27.0rc5
+0.27.0