Fix specification of dependencies

[StevenMaude]

I created #91 and found that the tests were failing. This is because of dependencies not being more rigorously specified, and two dependencies thus breaking backwards compatibility:

• pyhamcrest is now a v2
• xlrd is now a v2

I've patched it up here solely to get the tests to pass.

pyhamcrest is less important as it's a test dependency only. But xlrd is required, and its version is specified in the messytables setup.py, which itself is not pinned.

So this task involves:

• fixing up dependencies in the sensiblecodeio messytables fork
• fixing up dependencies here

[StevenMaude]

See the changes in #91.