Update JVM source/target version to 1.7

For making it compilable: java.nio.file.Files is from 1.7
Merge branch 'bugfix_update_jvm_version' of https://github.com/nahi/heroku-maven-plugin into nahi-bugfix_update_jvm_version
Merge branch 'nahi-bugfix_update_jvm_version'
added a change log file