update travis config and fix some test failures

In the travis config, drop explicit python-numpy from apt-get install list as
we want the version pre-installed in python site-packages. Installing
python-scipy will pull in the numpy .deb anyway, but the one in site-packages
will override it.

Fixed a doctest in integrate.py to reflect a certain numpy exception's new
message which was updated around v1.9. Technically this means that test will
now fail on older numpy versions.

In hello_pysb.py the imports required for simulation and plotting were moved
from the top level to the __main__ block, meaning the model can be imported
without scipy or matplotlib installed (minor but good practice).
Merge pull request #108 from jmuhlich/ci_testing_fixes

update travis config and fix some test failures
refactor setup.py get_version logic

Old git-related code moved to get_git_version, and code previously inlined
in main moved to get_version. Also it now looks for the RELEASE-VERSION
file relative to where setup.py itself lives, so calling it from another
directory will now work (important for getting sphinx doc build to work
from an sdist).
include docs and tests in sdist

fixes #99
Merge pull request #107 from jmuhlich/improve_sdist

Improve sdist