add BOOSTRAP_LATEX option to bootstrap command to install TeXlive on Ubuntu

- LaTeX is installed if BOOTSTRAP_LATEX environment variable is set
    - Ubuntu: use list of packages provided by Dirk
    - OS X: install basic version of MacTeX package (BasicTeX)
        - full version cannot be installed due to insufficient disk space (travis-ci/travis-ci#1586)
        - use explicit Utah mirror recommended by MacTeX site, seems to be fastest
        - remove package files after installation

- add test for BOOSTRAP_LATEX option in build matrix
Make mac+latex easier.

This makes two tweaks to Kirill's hard work:
* Add a base set of tex packages, so that we can use this for testing out of
the box.
* Change the way we handle paths, so that users don't have to source anything
in their `.travis.yml` unless they're doing something fancy (like installing
more of their own packages).
Merge branch 'kirill-merge'