don't add eol whitespace on blank lines in license

Simple change to avoid empty whitespace at the end of the line when there is a blank line in the license.

```
$ sbt scripted
Running projects / add-license-already-there
Running projects / add-license-from-inline
Running projects / add-license-wrong-one-there
[success] Total time: 38 s, completed Nov 24, 2014 10:37:27 AM
```