Avoid member vs. property name collisions in JRubyJar

[rtyler]

When using a member variable that is named the same as a property, the behavior
from subclassing can be very confusing

Take the following example:

    class Parent {
        protected String configuration = "foo"

        String getConfiguration() {
            return configuration
        }

        void doSomething() {
            logger.info("Parent using ${configuration}")
        }
    }

    class Child {
        String getConfiguration() {
            return 'hello'
        }

        void doSomething() {
            logger.info("Child using ${configuration}")
            super.doSomething()
        }
    }

Invoking childInstance.doSomething() will output:

Child using hello
Parent using foo

This can be remedied (pointed out by @ysb33r) by calling this.@configuration
which will force a property lookup in Groovy. I personally find it less
error-prone to keep member names ddifferent from the proprty names which expose
them as a non-protected/private API.

Fixes jruby-gradle/jruby-gradle-storm-plugin#21

[ysb33r]

Looks good to me

[rtyler]

The test failures were due to a copy paste error, accidentailly deleted the mainClass property when shuffling things around prior to committing. Hooray CI!