Podspecs for Protoc and the gRPC plugin

The examples under <repo_root>/examples now rely on the released
versions. Those under src/objective-c/examples, as well as the tests,
rely on protoc and the plugin as compiled from head.
diff --git a/tools/run_tests/run_tests.py b/tools/run_tests/run_tests.py
index b2cc213..ccfbfbe 100755
--- a/tools/run_tests/run_tests.py
+++ b/tools/run_tests/run_tests.py
@@ -634,7 +634,7 @@
     return []
 
   def make_targets(self):
-    return ['grpc_objective_c_plugin', 'interop_server']
+    return ['interop_server']
 
   def make_options(self):
     return []