Refactor build script to include travis build matrix

I believe the cause of these two build issues:

https://github.com/pixie-lang/pixie/pull/129 and https://github.com/pixie-lang/pixie/pull/131 

Were due to neither the build script nor the Travis builds building all the possible configurations (e.g. with/without JIT, with/without preload). I thought we could use Travis to build all the different versions for us.

This PR refactors out the build options into environment variables so we can use them in a Travis build matrix. Here's a screen shot of Travis building all the configs in a matrix with all the tests passing:

http://i.imgur.com/MOPR3y0.png