Adapt to latest ddmlib version

This change is a quick adaptation to changes in DDM lib used by Android Gradle plugin 0.14.x.
Closes #208.

That said, I do not have a strong leg to stand on what assumption failure really means. However, it seems to correspond to `error` in previous versions.
I didn't add ignored test handling since it requires additional changes in reports code.

I tested this with combination of spoon gradle plugin and Android plugin 0.14.1.
Assumption failure comes from JUnit 4. It means that the parameters in which the test was executing render the test irrelevant. It's much more like a programmatically determined ignore and should be treated as such.