Update CHANGELOG.md
Release: version 2.4.0
Release: version 3.0.0
Release: version 3.1.0
Merge branch 'master' of https://github.com/mopub/mopub-android-sdk

Conflicts:
	mopub-sdk/build.gradle
	mopub-sdk/src/main/java/com/mopub/mobileads/MoPubView.java