* FIX: install on python 3.7
https://github.com/python/cpython/pull/46 /
https://bugs.python.org/issue29463 move the module comment into the
AST node and hence out of the tree which means the 2nd entry in the
tree is now the import rather than the `__version__` string.
Adds nightly on travis.
* BLD: update python tags in setup.py
* CI: switch to 3.7-dev
* CI: allow failure on 3.7 dev
