Fix classpath unit tests

This fixes unit test errors caused by different path separators on Windows operating systems.

I was getting two unit test failures trying to build this project on Windows:

```
:test

org.robolectric.gradle.RobolectricPluginTest > ensureJarDependenciesOnClasspath FAILED
    java.lang.AssertionError at RobolectricPluginTest.groovy:274

org.robolectric.gradle.RobolectricPluginTest > ensureAarDependenciesOnClasspath FAILED
    java.lang.AssertionError at RobolectricPluginTest.groovy:290
```

Project now builds fine on Windows 8.1 and Ubuntu 14.10
I think it is better to use `File.separator` here? Somethink like `"junit${File.separator}junit${File.separator}4.8"` should work, too...