#760 Fixes for JDK 6 build

Two issues addressed here:

1. Some of Hamcrest's generics fail on JDK 6 but not on the more precise compiler of JDK 7. In particular, JDK 6 gets confused with the return type of `Matchers.contains()`, which is `Matcher<? extends T>` in conjunction with `Matchers.allOf(Matcher<? super T>)`. `Matchers.hasItems()` returns `Matcher<T>` which works in JDK 6. I converted it where it didn't change the meaning of the test.

2. In the maven invoker, an older reference of `jcabi-aspects` failed because of https://github.com/jcabi/jcabi-aspects/issues/21. I updated jcabi parent to suit.

Unfortunately we can't do much to test against the first scenario happening again in the future, except to watch for pull requests where JDK 6 doesn't want to compile the unit tests on Travis.