Remove guava as a dependency.

The only thing used from guava was Preconditions.java. With this, that file is copy/pasted and Android consumers are happy to be under the dex limit again.

Fixes: https://github.com/snowplow/snowplow-java-tracker/issues/86
Merge branch 'hamidp-hamidp/remove-guava' into release/core-0.2.0