OK. Then we need to drop java7 support totally in upcoming version.

@komamitsu @frsyuki
Should we bump the version number to 0.9.x before dropping Java7 support?

msgpack-java already use sbt 1.x (which does not support Java 7)

@xuwei-k I'm not sure how sbt handles target:jvm-1.7 option, but msgpack-core-0.8.14.jar still built for JVM7 (class version 0.51):

javap -cp . -v "org.msgpack.core.MessagePack" | grep version                                                                     [11:24:44 Feb 07 2018]
  minor version: 0
  major version: 51

but msgpack-core-0.8.14.jar still built for JVM7 (class version 0.51):

Yes, sbt 1.x can generate JVM7 byte code from Java sources. but sbt 1.x and Scala 2.12 compiler does not run on JVM 7.
This pull request changes scalacOptions not javacOptions. scalacOptions is different from javacOptions.
This change has no effect (just suppress warnings) because Scala 2.12 compiler says follows when set -target:jvm-1.7

[warn] -target is deprecated: -target:jvm-1.7 is deprecated, forcing use of jvm-1.8

@xuwei-k Thanks for making it clear. I got confused scalacOptions and javacOptions.

Merged.