Added missing import statement

I'm new to Java but I think this statement is required to solve "insufficientArgumentsToStringFormatCausesAnError" in a way consistent with the previous examples. Please disregard if my solution is incorrect.

```
    @Koan
    public void insufficientArgumentsToStringFormatCausesAnError() {
        try {
            String.format("%s %s %s", "a", "b");
            fail("No Exception was thrown!");
        } catch (Exception e) {
            assertEquals(e.getClass(), MissingFormatArgumentException.class);
            assertEquals(e.getMessage(), "Format specifier 's'");
        }
    }
```