RFR(XS): 8232713: Update BCEL version to 6.3.1 in license file

[Aleks Efimov]

Hi,

The BCEL version has been updated to 6.3.1 by JDK-8224157 [1]. The BCEL 
license file needs to be updated to include the correct version.

Webrev: http://cr.openjdk.java.net/~aefimov/8232713/00

JBS: https://bugs.openjdk.java.net/browse/JDK-8232713


With Best Regards,
Aleksei

[1] https://bugs.openjdk.java.net/browse/JDK-8224157