Set Java execution environment to 1.8

[PeterJohnson]

With the introduction of JDK 9, it's possible teams will start trying to build robot code with it. The roboRIO only has JDK 8. It appears this can be done with .settings/org.eclipse.jdt.core.prefs in the project folder with the following settings:

eclipse.preferences.version=1
org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.8

[sciencewhiz]

I think we probably also want

org.eclipse.jdt.core.compiler.compliance=1.8
org.eclipse.jdt.core.compiler.source=1.8