Fix subject-verb agreement in error msg (and other grammar stuff)

Before: 
```
pending_migrations = [20130417223021]

> rake db:test:prepare
>> You have 1 pending migrations. 
```

After:
```
pending_migrations = [20130417223021]

> rake db:test:prepare
>> You have 1 pending migration. 
```

(And 2 other small grammatical errors.)

Note, I didn't see tests for these output messages. And for the pending migrations error, the only way I could think of to test it was to break it out into a separate method, which I didn't think was worth it. 

Feedback welcome.

(I also didn't think this warranted a changelog entry.)