Fix issue with flexible rail return value

Between 1.6 and 1.7 the return value to BlockRailBase.isFlexibleRail()
was inverted. While this is not a huge deal and could be worked around
by simply inverting your return value, it does mean its no longer
consistent with the function name and javadocs.
Merge pull request #1050 from CovertJaguar/master

Flexible Rail return value was inverted between 1.6 and 1.7