Use both JDK7 and JDK8 in Travis
Fix javadoc compilation errors on jdk8
Merge pull request #1168 from nbelikov/1163_javadoc_for_jdk8

Fix JavaDoc compilation errors on JDK8
Merge pull request #1164 from WonderCsabo/travisJdk8

Use both JDK7 and JDK8 in Travis
Only download the binary distribution of Gradle wrapper
fix: generic issues in EFragment
Merge pull request #1117 from Artyomcool/1077-fragment-generics

fix: generic issues in EFragment
Clean contentView_ reference in onDestroyView()

If setRetainInstance(true) has been set on the fragment or
addToBackStack() was applied to FragmentTransaction, the fragment won't be
destroyed and will keep the reference to its parent view in contentView_
and could lead to memory leak.

This change implements onDestroyView() which cleans the reference.
However, the developer is still responsible for cleaning up the injected
views, because otherwise a (badly implemented) background task may get a
NullPointerException.
Clean refs to injected views in onDestroyView()