TST: remove yields in coordinate_system tests

Remove use of generators in tests to make them easier to debug
RF: allow object coordinate in coordinate system

Objects as coordinate values can be useful for sympy expressions as
coordinates.  Objects can often be subtracted, compared etc.  In fact we
already use objects as a coordinate type in the documentation.
RF: allow coordinate map to work with object dtype

Coordinate maps can sensibly have object dtypes in the coordinate
system, such as sympy symbols.  Take these into account for inverses,
permutations.

The inverse of an AffineTransform previously could have a different
coordinate input / output type from the original.  Constrain to have the
same coordinate types, return None for inverse otherwise.

Adapt the doctest for __call__ to reflect this.
BF: allow CoordinateMap subclasses on L & R of ==

We were previously only testing subclasses in one direction, but a
subclass on the left or the right should be OK.  Test.
RF: refactor to restore previous inverse behavior

Restore the previous behavior of returning a floating point inverse
(mostly) from an int input AffineTransform.  Add flag to inverse method
to specify new behavior of returning None when no integer inverse can be
found.
BF: fix travis yml file for build errors

Travis seems to be timing out for the slow tests now.  Run the not slow
tests only.

The sympy build command was not correct and it has probably been
building the master up until now.  Fix to build the latest release.
Merge pull request #248 from matthew-brett/allow-object-coordinates

Allow object coordinates in CoordinateSystem and CoordinateMap