TST: Fix broken tests and make Travis run all tests.

TST: Put off fixing test_ci for now.

TST: Do not install mpl.

TST: Skip Nelder-Mead test if not present
Merge pull request #72 from danielballan/run-tests-on-travis

TST: Fix broken tests and make Travis run all tests.
Merge branch 'master' of https://github.com/lmfit/lmfit-py