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