Only build on jdk8

As of https://github.com/sbt/util/pull/66 sbt/util is built and
published using jdk8.

Trying to use jdk7 will fail with "Unsupported major.minor version
52.0", e.g. for xsbti/Logger.

.travis.yml

@@ -2,6 +2,4 @@ language: scala
 scala:
   - 2.11.8
 jdk:
-  - openjdk7
-  - oraclejdk7
   - oraclejdk8