add fs-events gem on osx/linux to make guard happy
add fs-events gem on osx/linux to make guard happy

Change-Id: I004fc6aba99cef99dcc8829ee674aec51e0bb3df
Reviewed-on: https://gerrit.instructure.com/6065
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>

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