Fix test failure regarding int->float conversion

Supply only small integers (< 1^24) to int->float conversion tests,
so they can roundtrip without error.

I got a failure building 0.5.9 in Fedora:
http://koji.fedoraproject.org/koji/buildinfo?buildID=543205
.```
[----------] 1 test from IntegerToFloatingPointTestInstance/IntegerToFloatingPointTest/0, where TypeParam = (anonymous namespace)::TypePair<float, long long>
[ RUN      ] IntegerToFloatingPointTestInstance/IntegerToFloatingPointTest/0.simple_buffer
msgpack_test.cpp:220: Failure
Value of: fabs(val2 - val1) <= kEPS
  Actual: false
Expected: true
```
The test supplies large integers which do not fit in the float significand and thus cause rounding error.  How about limiting the input values to 0 < x < 1^24?