pinning dependency versions

[ryancoe]

In #41, we started to pin specific versions for each dependency, but this is potentially problematic as it will lead to conflicts for users trying to use wecopttool with other packages (see, e.g., https://stackoverflow.com/questions/28509481/should-i-pin-my-python-dependencies-versions). For the time being, we will not pin versions for dependencies unless we observe the need to do so.

[ryancoe]

Note that if we ever did want to do this, bb424fa would be a starting point.