Fix failing build on CI by specifying pip package version (#329)

`bazel build //...` was failing due to "googleapis-common-protos[grpc]"
pip package being unavailable.

It seems to be caused by latest googleapis-common-protos release.
Specify googleapis-common-protos in requirements.txt to be in the previous
version (1.51.0) to fix this.

Fixes #321.

examples/extras/requirements.txt

diff --git a/examples/extras/requirements.txt b/examples/extras/requirements.txt
index 743bbe7..49b86eb 100644
--- a/examples/extras/requirements.txt
+++ b/examples/extras/requirements.txt
@@ -1 +1,2 @@
 google-cloud-language==0.27.0
+googleapis-common-protos==1.51.0