Add com.google.zxing:core:2.2 dependency to sample Gradle file

Fixes #3
Update Travis config for Android SDK installation.
Perhaps we do need `apt-get update` on Travis.
Update build tool versions.

Android build-tools 18.1.0.
Gradle 1.8.
Android Gradle Plugin 0.6.3.
Merge https://github.com/embarkmobile/zxing-android-minimal into dev
up to version 1.1.4