BLD: add requirements files

This commit adds 5 requirements files: one for each Travis build. The
corresponding sections in ci/install.sh are also removed in favor of these
files.

This allows pandas to have more fine-grained control over versions of
hard and soft deps when using travis.
CLN: do not depend on ordering of travis.yml
CLN: add explicit versions for everything
CLN: pass explicit path in ci/install.sh
BLD: add FULL_DEPS=true to python 3.3 travis config