Fix '*-ea' version string handling

I ran into an issue using an old "early access" version of Java 8 with version number "1.8.0-ea", which causes a NumberFormatException in [Sys.java#L97](https://github.com/RichardHightower/boon/blob/0ad64c28de9c514c74090b22e61f53dc0869e9dd/src/main/java/org/boon/core/Sys.java#L97). Seeing that "1.8.0" is handled as special case, I simply did the same for "1.8.0-ea".

I also noticed there's a substring expression where the return value is not used.