Update pom.xml to specify that test code is Java 8

To get the test case JvstTest4.testMethodParameters() to work
the <testSource> and <testTarget> elements are set to "1.8"
and the compiler argument "-parameters" is added.
Fix JASSIST-220

Static methods on interfaces in Java 8 means
that the INVOKESTATIC opcode can sometimes refer
to an item that is an InterfaceMethodrefInfo
(instead of a MethodrefInfo).

This is now handled in the
ConstPool.getMethodref...() methods by casting
to MemberrefInfo instead (since MemberrefInfo
contains all the needed information).
Merge pull request #11 from jkaving/JASSIST-220

Fix for JASSIST-220