bump up required version of bazel to 0.19

Change-Id: Id2b3e724c037eb94b5204537129c7c489ba4dd5c
diff --git a/tools/build/bazel/bazel_version.bzl b/tools/build/bazel/bazel_version.bzl
index 5f880e3..27835d0 100644
--- a/tools/build/bazel/bazel_version.bzl
+++ b/tools/build/bazel/bazel_version.bzl
@@ -16,12 +16,12 @@
 
 def check_bazel_version():
     if "bazel_version" not in dir(native):
-        fail("\nBazel version is too old; please use 0.15.* official release!\n\n")
+        fail("\nBazel version is too old; please use 0.19.* official release!\n\n")
     elif not native.bazel_version:
-        print("\nBazel is not a release version; please use 0.15.* official release!\n\n")
+        print("\nBazel is not a release version; please use 0.19.* official release!\n\n")
         return
 
     versions = native.bazel_version.split(".")
     if not int(versions[1]) >= 15:
-        fail("\nBazel version %s is not supported; please use 0.17.* official release!\n\n" %
+        fail("\nBazel version %s is not supported; please use 0.19.* official release!\n\n" %
              native.bazel_version)