#20 Write tests for Client module

[gk-per]

What I'm doing:

• write tests for client.ex module testing various response codes from API are handled correctly by client
• add yecc and leex to compilers to fix 2 warnings

What it looks like:

*[cs/client-test][~/src/prodops_ex]$ mix test
....
Finished in 0.01 seconds (0.00s async, 0.01s sync)
4 tests, 0 failures

What I'd like feedback on:

• What other modules should we be testing?