Summary
-------

Add a new enum constant `RELEASE_11` to `javax.lang.model.SourceVersion` for the JDK 11 release.

Problem
-------

The `SourceVersion` enum needs an enum constant for each release being modeled.

Solution
--------

Add the enum constant `RELEASE_11`.

Specification
-------------

    --- old/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java	2017-12-11 22:32:47.930062197 -0800
    +++ new/src/java.compiler/share/classes/javax/lang/model/SourceVersion.java	2017-12-11 22:32:47.586062182 -0800
    @@ -159,7 +159,15 @@
          *
          * @since 10
          */
    -     RELEASE_10;
    +     RELEASE_10,
    +
    +    /**
    +     * The version recognized by the Java Platform, Standard Edition
    +     * 11.
    +     *
    +     * @since 11
    +     */
    +     RELEASE_11;

Also, the output of `SourceVersion.latest()` is changed to `RELEASE_11`.