Added quotes when defining the precision for the `decimal` fields

before:

```bash
$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic}
```

after:

```bash
$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
```
Merge pull request #163 from mogetutu/master

Added quotes when defining the precision for the `decimal` fields