Fix warnings about JRE 1.6

A number of projects had the JRE System Library set to "JavaSE-1.6" which causes the following error message when only JDK 7 is installed: _"Build path specifies execution environment JavaSE-1.6. There are no JREs installed in the workspace that are strictly compatible with this environment."_

Since [we require JDK 7+](https://github.com/libgdx/libgdx/wiki/Setting-up-your-Development-Environment-%28Eclipse%2C-Intellij-IDEA%2C-NetBeans%29) most people will probably not have JDK 6 installed as well.

I have changed the JRE System Library to use "Workspace Default JRE" and instead enforced the JDK 1.6 Compiler Compliance level. (A number of projects were already set up that way) This eliminates the warning while still ensuring the use of Java 6 API only since Java 7 is unsupported before Android Lollipop.