REF/API: numba decorators only turn on for numba 0.11.*

[danielballan]

closes #81 and #82

[danielballan]

The purpose of this PR evolved after an in-person conversation between Nathan and me.

If numba 0.11.* is installed, numba-accelerated mode is enabled by default.

If numba is not installed or a version other than 0.11.* is installed, numba is disabled by default. If you try to use engine='numba' or call trackpy.enable_numba(), you will get an informative error message, mentioning which version of numba (if any) trackpy sees and which version it wants.

We will probably never support 0.12, but we will revisit for numba 0.13+ when the project is (we hope) more stable. See links in #82 for discussion on what happened over at numba. Since numba 0.12 is (apparently accidentally) the new default with conda, I added a Travis build to make sure trackpy is falling back to pure Python and working properly with out-of-the-box conda, even though numba 0.11 is recommended in the docs.

@nkeim please merge if this looks good to you.

[nkeim]

Great! This will get us through numba's growing pains. Thanks!