Update NuGet packages to latest versions.
Revert "Prevent Progurad from stripping GCM functionality."

This reverts commit 7242924c9dad91fc066d364e02d2dbc2475aecb4.

Closes #156.
Update Google Play Services to 19.0.0.
Revert "Workaround support library ART bugs."

This reverts commit df474ab14487064737bab289480d0839c14443c0.

Closes #126.

Conflicts:
	Joey/UI/Fragments/ChooseTimeEntryTagsDialogFragment.cs
Bump target framework to 20.

This is for XamGuard to work correcty as the new Google Play Services
library references APIs from framework 20.
Merge pull request #295 from toggl/roosmaa-update-packages

Update packages & components.