Podfile.lock prevents gRPC to be properly installed

When Podfile.lock is present, the prepare_command of the gRPC pod isn’t
executed on “pod install”, even if the Xcode workspace and the Pods/
directory are absent.

src/objective-c/examples/Sample/Podfile.lock

-PODS:
-  - gRPC (0.0.1):
-    - gRPC/C-Core (= 0.0.1)
-    - gRPC/GRPCClient (= 0.0.1)
-    - gRPC/ProtoRPC (= 0.0.1)
-    - gRPC/RxLibrary (= 0.0.1)
-  - gRPC/C-Core (0.0.1):
-    - OpenSSL (~> 1.0.200)
-  - gRPC/GRPCClient (0.0.1):
-    - gRPC/C-Core
-    - gRPC/RxLibrary
-  - gRPC/ProtoRPC (0.0.1):
-    - gRPC/GRPCClient
-    - gRPC/RxLibrary
-  - gRPC/RxLibrary (0.0.1)
-  - OpenSSL (1.0.201)
-  - ProtocolBuffers (1.9.8)
-  - RemoteTest (0.0.1):
-    - gRPC (~> 0.0)
-    - ProtocolBuffers (~> 1.9)
-  - Route_guide (0.0.1):
-    - ProtocolBuffers (~> 1.9)
-
-DEPENDENCIES:
-  - gRPC (from `../../../..`)
-  - RemoteTest (from `RemoteTestClient`)
-  - Route_guide (from `RouteGuideClient`)
-
-EXTERNAL SOURCES:
-  gRPC:
-    :path: ../../../..
-  RemoteTest:
-    :path: RemoteTestClient
-  Route_guide:
-    :path: RouteGuideClient
-
-SPEC CHECKSUMS:
-  gRPC: f6c1bf5dde59ab543e4bd1d5e2ea56da4a9a0253
-  OpenSSL: 4e990d04b14015c49c800c400b86ae44a4818a5c
-  ProtocolBuffers: 9a4a171c0c7cc8f21dd29aeca4f9ac775d84a880
-  RemoteTest: 021a51c04d5795f286b379ca5ef14d0be5b2fb9b
-  Route_guide: a277da8eef182774abb050d7b81109f5878f8652
-
-COCOAPODS: 0.36.0