This updates the readme to specify the JDK version used by the project to be at least JDK8. Let me know if we need JDK7 builds after all, my opinion is just based on what is inside the pom.xml and other build configurations.

Tested JDK7 using Travis - test failed, due to "[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project aws-dynamodb-encryption-java: Fatal error compiling: invalid target release: 1.8 -> [Help 1]"

Updated .travis.yml to JDK8 and tested on Travis - tests passed

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