install testthat from GitHub

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~~