SemaphoreCI support

[slowr]

Added support for SemaphoreCI by parsing corresponding environment variables.

To avoid pylint failure:

R:103, 4: Too many return statements (7/6) (too-many-return-statements)

I changed the behavior of load_config_from_ci_environment to return only once in the end.

[slowr]

To address the failed checks:

• Coveralls reporting a coverage decrease refers to a file that I have not changed
• Travis-ci failures seem to happen randomly

[TheKevJames]

Thanks for the contribution! Looks good to me. Sorry about the CI issues, looks like a bit of weirdness on the Coveralls API side of things.