[xray] Fix compilation on mac

[pcmoritz]

On my mac I get the following:

/Users/pcmoritz/ray/src/ray/object_manager/object_directory.cc:151:8: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
      [this, callback](gcs::AsyncGcsClient *client, const ObjectID &object_id,
       ^
1 error generated.
make[2]: *** [src/ray/CMakeFiles/ray_objlib.dir/object_manager/object_directory.cc.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [src/ray/CMakeFiles/ray_objlib.dir/all] Error 2
make: *** [all] Error 2

(introduced by #2162)

[AmplabJenkins]

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[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/5908/
Test PASSed.