travis.yml build matrices

[parul-l]

The .travis.yml file needs two fixes:
(1) the matrix with the PYPI_BUILD environment variables is not being included for each of the four environments we have set up. From the travis docs, we probably want something similar to the example

language: ruby
rvm:
- 1.9.3
- 2.0.0
- 2.1.0
env:
- DB=mongodb
- DB=redis
- DB=mysql

and then have an exclude portion for the osx builds.

(2) in the install portion, we only want to install the required plotting packages if the PYPI_BUILD variable is complete or only_plotting