Update "trunk" to require Java 8

Since "trunk" is unreleased as of yet, and Java 8 is quite mature, and Java 9 is due out later this year, and there are features of Java 8 (such as lambdas) that we would like to take advantage of for new development, it seems wise to require Java 8 going forward for the "trunk", while leaving the 2.0.x maintenance branch at Java 6. Previously we had upgraded to requiring Java 7 for "trunk", but that version is stabilized and no longer being updated...