As a continuation of some of the discussion over at #123, this pull request removes the dependency on JDK6 & 7 installations to successfully compile the project with Maven.

Previously, JDK 6/7 were needed for their rt.jar in bootclasspath to ensure that no new classes/methods/fields/code above the target language level were accidentally introduced during development, causing potential runtime errors for library users. With the successful merge of #124, JDK 9+ is now required for project compilation, which enables support for javac's --release flag that implements the same functionality without any dependency on JDK 6/7. This feature was not possible in the previous required JDK 8.

Impacts:

• Appears to consistently shave off roughly ~8 seconds from fetching JDK 6 during CI builds, observed from the build times in prior Travis CI builds.
• Requires a single JDK toolchain to compile this project, rather than 3 separate (and explicitly configured) JDKs. Although there has not been a much recent pull request traffic, this should improve the workflow for developers making future pull requests. Notably, it took me a non-negligible amount of time to realize I needed JDK 6 & 7 to author Experimental JDK9 module-info support #124, as rt.jar was removed from JDK 9 and this caused builds to immediately fail with the default properties.