Bump required Bazel version to 2.1.0

Change-Id: I2e4f44a8d591ba26c14189fc46ead0feaef187c3
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 3eefcb9..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-1.0.0
+2.1.0