Speed up the tests

[pwl]

At this point the Travis tests take 16 min to complete, we should decrease this time significantly. The biggest issue are the OS X builds as they take 2x time to complete as Linux builds. We could also consider choosing the parameters to better suite the test cases (tolerance, initial steps etc.) and separating the tests for legacy solvers (ones without iterators) from the new solvers. For the latter the tests in test/iterators.jl cover the same test cases but run significantly faster on my machine.

[mauro3]

I think the main problem is the build time, not the tests themselves. To reduce that we may need to ditch some packages.

[pwl]

What do we need Polynomials for?

[mauro3]

Adam-Bash