Use a value larger than Fixnum max to test Bignum support.

On 64 bit system the previous test value 1234567890 still fits in a Fixnum
so the tests will fail. Use a dynamically calculated value instead to ensure
that the value will force a Bignum in all cases.

Fixnum maximum calculation from
http://stackoverflow.com/questions/535721/ruby-max-integer
Merge pull request #18 from graaff/long-tests-64bit

Use a value larger than Fixnum max to test Bignum support.