Added ignoreFailures to DSL

This mirrors another Gradle `Test` behaviour to the Robolectric Plugin (see http://www.gradle.org/docs/current/dsl/org.gradle.api.tasks.testing.Test.html)

If `ignoreFailures true` is set in the DSL, failed tests will not fail the build.

I also updated the version and Gradle Build tools to `0.12.0`.