Update tests

[dschep]

• Don't skip non-docker tests when docker isn't present
• Don't hard code a static cache md5sum

@AndrewFarley, I noticed that tests were failing on #238 and got the same errors locally on master. Do you think it's reasonable to compute the md5sum this way? I don't like hard coding the hash. But, I understand that this makes the test verify less of the process. Best would probably be to externally replicate what genrerateRequirementsFile does to create a new file and md5sum that instead of .serverless/requirements.txt

[dschep]

I've updated to replicate the requirements file generation too (required a few tweaks: leave out blank lines and add a new line at the end of the file as is standard in *nix environments) so that this doesn't rely on .serverless/requirements.txt for md5sums

[AndrewFarley]

@dschep This looks like a good fix, and would keep things from breaking in the tests if/when that requirements file changes. +1 from me on this