Improve coveralls-reported build coverage

[khaeru]

Coveralls.io currently gives 25% coverage for the py-gdx codebase/test suite—about 100% in gdx/test/test_gdx.py and zero elsewhere. On the other hand, local testing gives:

- coverage: platform linux, python 3.5.2-final-0 -
Name                Stmts   Miss  Cover
---------------------------------------
gdx/__init__          232     27    88%
gdx/api                45      0   100%
gdx/pycompat            7      0   100%
gdx/test/test_gdx     101      2    98%
---------------------------------------
TOTAL                 385     29    92%

Fix so that the travis/coveralls-reported coverage matches.