install testthat from GitHub

[krlmlr]

so that tests are actually run on Travis (they were simply skipped before)

• example for code with a failing test where no error is reported: https://travis-ci.org/krlmlr/devtools/builds/14834370
• after applying the patch (actually, reporting failure of something else): https://travis-ci.org/krlmlr/devtools/builds/14835799
  • this pull request currently also fails, but includes code to dump logs

[hadley]

Why do you want to do this?

[krlmlr]

Cargo cult. We may as well leave it out if you prefer.

[krlmlr]

Travis shows errors because texi2dvi (i.e., LaTeX) is not installed. Do we want to install LaTeX for four tests, or do we just skip them if texi2dvi is not available?

[krlmlr]

Ready for merging. Added LaTeX for now so that the current codebase doesn't throw errors.