memorize the calculated file dependencies. Thanks to Ignacio Piantanida for the patch.

http://hipbyte.myjetbrains.com/youtrack/issue/RM-466

* before
  % time rake
  ... snip ...
  rake  38.41s user 2.77s system 101% cpu 40.515 total

* after
  % time rake
  ... snip ...
  rake  18.01s user 1.64s system 89% cpu 21.877 total
use a cached file dependencies in order to reduce the build time

http://hipbyte.myjetbrains.com/youtrack/issue/RM-466

* before
  % time rake
  ... snip ...
  rake  18.01s user 1.64s system 89% cpu 21.877 total

* after
  % time rake
  ... snip ...
  rake  2.74s user 1.41s system 119% cpu 3.460 total
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