jvmci-0.15 release

[Doug Simon]

Due to a number of issues identified by FindBugs[1] when run over the JVMCI code in JDK9, a number of JVMCI changes had to be made, including API changes. These changes have already been pushed to graal-jvmci-8[2] along with a new tag (i.e., jvmci-0.15). The graal-core changes that adopt the JVMCI API changes are also pushed[3][4].

-Doug

[1] http://findbugs.sourceforge.net
[2] http://hg.openjdk.java.net/graal/graal-jvmci-8/rev/abebb99b6b50
[3] https://github.com/graalvm/graal-core/commit/9489b088c704c3f323f973a98338cf3300492271
[4] http://hg.openjdk.java.net/graal/graal-core/rev/190661a57365