Fix the mmap resize to use the correct size.

Fixes #54. Previously the DB was calculating a minimum mmap size but
using the wrong variable after it calculated the size. This commit
changes the DB to use the correct variable.
Merge pull request #55 from benbjohnson/fix-mmap-resize

Fix the mmap resize to use the correct size.
Merge branch 'master' of https://github.com/boltdb/bolt