Travis

Some small modifications to the Travis build. As it was before, a failure of an assertion in the travis_post script would not result in a recorded build failure. See http://docs.travis-ci.com/user/build-configuration/#Build-Lifecycle.

I also added the assertion that the pdf of extra labs to the trais_post script. Technically I don't think these tests for file existence are necessary since we'll see pdflatex fail first, but it is good to have a script in place for post-compile tests.