travis-ci: sphinx needs to have things in path
travis-ci: sphinx needs to have things in path

To build the sphinx/python doc, The order of the make, needs to be:
 - cmake -DPYTHON_BINDINGS=ON
 - make
 - make install
 - cmake -DPYTHON_BINDINGS=ON -DWITH_DOC=ON
 - make

Make it so.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio