Version dropdown in docs

This change makes the version dropdown work correctly, by defaulting to the version of the docs you're actually navigating to. Deploying this change requires updating all of the past releases docs, i've been using this script locally.

The idea is to check out the old versions from git, but get the _new_ sphinx theme including this change, and then rebuild. This should be a one time thing -- new releases shouldn't need to change the HTML for the old docs, except for this one time.

```
rmcgibbo@vspm9  ~/local ()
$ cat build-old-docs.sh
set -x
rm -rf docs-output
mkdir -p docs-output
wd=`pwd`

cd mdtraj
RELEASES=`wget -qO- https://api.github.com/repos/SimTk/mdtraj/releases | jq -r '.[] | .tag_name '`
for r in $RELEASES;
do
    conda create -n docs-build --yes numpy scipy cython openmm ipython-notebook pip matplotlib scikit-learn sphinx==1.2.3 pytables nose pandas
    source activate docs-build
    pip install numpydoc runipy==0.0.4

    git clean -xdf
    git reset --hard $r
    git checkout origin/js-version-docs docs/conf.py
    git checkout origin/js-version-docs docs/themes
    git checkout origin/js-version-docs docs/sphinxext
    python setup.py install

    cd docs
    make html
    mv _build $wd/docs-output/$r
    cd -

    source deactivate
    conda remove -n docs-build --all --yes
    exit

done;
```