Fixing exception during tests

There was an exception when running tests caused by a lack of a ring
dependency. I added that in in the project.clj file and restructured the
:require line for consistency in core.clj
Modifying average function to take a list as a parameter
Changing == to = in if statement
Adding tests for the addValue method
Changing the POST method to parse the added value and return just the modified list
New tests for getting the average for a given key
Writing function to get average for a key, modify response to fit instructions, and return nil instead of 0.0 for keys that don't exist
Adding new tests for getting the average of all averages
Writing function to get average of averages for all keys