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

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 49ab185..6765f24 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -6,7 +6,7 @@
 
 default_java_toolchain(
     name = "error_prone_warnings_toolchain",
-    bootclasspath = ["@bazel_tools//tools/jdk:platformclasspath9.jar"],
+    bootclasspath = ["@bazel_tools//tools/jdk:platformclasspath.jar"],
     jvm_opts = JDK9_JVM_OPTS,
     package_configuration = [
         ":error_prone",