Require minimum Bazel version 0.19.0

Test plan:

    bazel build //...
    bazel test //...
    bazel build --java_toolchain //tools:error_prone_warnings_toolchain //...

Change-Id: I6c1256c20aa0ba4e7308f3dfcc68d8b8e53b1877

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 466325d..2c91a4a 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -11,7 +11,7 @@
 
 load("@bazel_skylib//:lib.bzl", "versions")
 
-versions.check(minimum_bazel_version = "0.17.1")
+versions.check(minimum_bazel_version = "0.19.0")
 
 load("//tools:bazlets.bzl", "load_bazlets")