Use JDK8 in Travis

It has improved memory-management for example the whole PermGen space
has been replaced with the new MetaSpace. We thought maybe some OOM
causes our test to fail in Travis, so that's why i made this change,
but it did not help. This is still interesting, because it is a good
thing to run the build and the test on a newer VM.
Update maven plugin versions

So they are working well even with JDK8. Precisely, the old version of
the findbugs plugin has issues with JDK8. Also added maven version
requirenment to the POM, so we can properly display newer versions with
versions:display-plugin-updates. Moreover, the build will fail if
someone tries to use a lower maven version.
Update build-tools in Travis
Fix stalling usage statistics dialog

The dialog is also opened if there was no pingId saved to
~/.android/ddms.cfg. See
https://android.googlesource.com/platform/sdk/+/master/eclipse/plugins/com.android.ide.eclipse.adt/src/com/android/ide/eclipse/adt/internal/welcome/AdtStartup.java
line 84.
It is important to close this dialog, since it can hang the test due to
waiting for user interaction.