Fix build failure

The recent build failures caused were caused by the new tests in PR #129 trying to use a file (committees-current.yaml) that didn't exist.

The solution is to first clone the relevant git repo to ensure the latest version of the file exists:

     os.system("git clone -q --depth 1 https://github.com/unitedstates/congress-legislators congress-legislators")

(An alternative solution would be to wget the single file from https://raw.githubusercontent.com/unitedstates/congress-legislators/master/committees-current.yaml.)