Ensure that Swift source files terminate with exactly one newline character

Swift source files should terminate with exactly 1 `\n` character, not 0 nor more than 1.
**Preferred**

``` swift
let myConstant = 42¬
```

**Not Preferred**

``` swift
let myConstant = 42
```

``` swift
let myConstant = 42¬
¬
¬
```
