Explicitly define minamal Java version 1.5 for Maven compilation

Hi,

until recently the default Java version used by Maven if nothing else was specified was 1.3. Although the maven-compiler-plugin switched to 1.5 now, my Maven 2 and 3.0.4 installations are still unable to compile purejavacomm out-of-the-box. Therefore, I propose the following fix which explicitly defines the minimum required Java version 1.5, so  compilation works more independent from the actual Maven version.

Thanks,
 -Marco