pool_test is flaky on OSX 10.9.5

It looks as if `pool_test` is not 100% reliable, at least not on my mac. Sometimes everything passes, but most of the time there is a single assertion the fails.

```
-------------------------------------------------------------------------------
TestPoolPointers
-------------------------------------------------------------------------------
/Users/zack/Dev/entityx-build/entityx/help/Pool_test.cc:45
...............................................................................

/Users/zack/Dev/entityx-build/entityx/help/Pool_test.cc:65: FAILED:
REQUIRE( extrapolated_p16 != static_cast(p16) )
with expansion:
0xb05540d9d67f0000 != 0xb05540d9d67f0000

===============================================================================
3 test cases - 1 failed (13 assertions - 1 failed)
```

Running `make test` over and over will yield success sometimes, but failure most of the time. This is built using Apple clang:

```
-- The C compiler identification is AppleClang 5.0.0.5000279
-- The CXX compiler identification is AppleClang 5.0.0.5000279
```

I thought that the failures were related to a change I had in a local branch but it turns out that this bejavior is also present in the current master.