#39: ignore columns with primaryKey set to false when determining foreignKeys

[janhapke]

Here is the fix together with the unit tests which no longer fail.

[devinivy]

@particlebanana node v5 consistently choking on the integration tests for some reason. PR looks good to me 👍.

Running integration tests...
rm -rf node_modules/waterline-adapter-tests/node_modules/waterline;
ln -s "/home/travis/build/balderdashy/waterline-schema/test_integration/waterline-master" node_modules/waterline-adapter-tests/node_modules/waterline;
ln: failed to create symbolic link `node_modules/waterline-adapter-tests/node_modules/waterline': No such file or directory
make[1]: *** [test-integration] Error 1
make[1]: Leaving directory `/home/travis/build/balderdashy/waterline-schema/test_integration/waterline-master'
npm ERR! Test failed.  See above for more details.
make: *** [test-integration] Error 1
npm ERR! Test failed.  See above for more details.