install libharfbuzz-dev and libfribidi-dev for the new textshaping pa…

…ckage (although I have no idea what it is), which is required by ragg, which is suggested by knitr

.travis.yml

@@ -29,6 +29,8 @@ addons:
     - libmagick++-dev
     - libpoppler-cpp-dev
     - libglu1-mesa-dev
+    - libharfbuzz-dev
+    - libfribidi-dev
 
 repos:
   XRAN: https://xran.yihui.org