simplify some code, make tests more 4.1-friendly
oops variable shadowing /o\
fix invalid HABTM foreign key in rails 4.1, fix tests. closes #12

immigrant was actually mostly 4.1 compatible, except that HABTM now
creates an extra (and invalid) association, causing an invalid FK

tests were aploding all over under 4.1, due to the above, and due to
HABTM now creating anonymous AR descendants (which also led to
namespaced test classes being inferred incorrectly)