Fix shared data problem on MacOS 10.9 with xcode 5.1

Move to construct on first use idiom for singleton definition,
which prevents problems with singletons between ruby swig modules
in an environment with multiple modules on MacOS 10.9 with xcode 5.1.

Before this fix, data was being shared between modules which caused
a crash on shutdown of the ruby interpreter if more than one
module was loaded at a time.