Allow JDK 7 to fail in build matrix

Some OpenJDK 7 and Oracle JDK 7 releases in the wild are broken with
respect to invokedynamic. This is affecting other projects like Groovy.

This introduces a Travis-CI build matrix where Oracle JDK 8 is the sole
reference. This will also let us know if JDK 7 releases get eventually
fixed at some hypothetical point in the future. Who knows?