Fix conj* and extend* contracts to expect collections

[countvajhula]

Fixes #23 by modifying the expected return values in the conj* and extend* contracts to be collection? instead of sequence? [the docs confirm that this is the intended output]

[coveralls]

Coverage remained the same at 89.421% when pulling 27573669c4bfca88c917fdfc421b2e5dd9624816 on countvajhula:fix-conj-extend-contracts into c4822fc200b0488922cd6e86b4f2ea7cf8c565da on lexi-lambda:master.