File#utime: Don't check for usec resolution on Darwin

[andre-richter]

Fixes failing specs discussed in #3502

andre@MacPro:~/rubinius$ bin/mspec -tr spec/ruby/core/file/utime_spec.rb
ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]
...

Finished in 0.001898 seconds

1 file, 3 examples, 10 expectations, 0 failures, 0 errors
andre@MacPro:~/rubinius$ bin/mspec -tx spec/ruby/core/file/utime_spec.rb
rubinius 2.5.8.c106 (2.1.0 4de8eb50 2015-10-16 3.5.1 JI) [x86_64-darwin14.5.0]
...

Finished in 0.005834 seconds

1 file, 3 examples, 10 expectations, 0 failures, 0 errors

[yorickpeterse]

Thanks!