use database_rewinder instead of database_cleaner

DatabaseRewinder is an alternative to DatabaseCleaner that implements a memoization strategy for only deleting tables which had INSERT's performed on them.

I've swapped out database_cleaner for database_rewinder on a number of projects and I've been happy with it.

Appears to shave a good 10-20 seconds off the test suite runtime:

```
$ time bundle exec rake
```
### master
```

real    1m9.777s
user    0m57.120s
sys     0m5.202s

```
### database-rewinder
```
real    0m51.393s
user    0m41.336s
sys     0m4.259s
```