Bump required Bazel version to 2.1.0

Change-Id: I79d391dd1c6a411db0ab11e674186373450600b8

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 26aaba0..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-1.2.0
+2.1.0