Bump Bazel version to 3.1.0

Change-Id: Id58d1603846a88be3493165583adca481c026eed
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0