all: switch to jdk8 as default

all: switch to jdk8 as default

Also, remove jdk7 as it will become unused. We added jdk8 for
retrolambda, and now that we will use jdk8 as the default, jdk7 is
unnecessary as retrolambda can work fine with just jdk8.

This removes it from the buildserver, and the new CI image also only has
jdk8 from jessie-backports.

Fixes #185.