[travis] Install libicu

.travis.yml

@@ -4,9 +4,10 @@ compiler:
   - gcc
 install:
   - sudo apt-get install pkg-config ragel   # for autogen.sh
-  - sudo apt-get install libfreetype6-dev   # for font fallbacks
-  - sudo apt-get install libglib2.0-dev     # for tests / utils
+  - sudo apt-get install libfreetype6-dev   # for font functions
+  - sudo apt-get install libglib2.0-dev     # for font functions / tests / utils
   - sudo apt-get install libcairo2-dev      # for utils
+  - sudo apt-get install libicu-dev         # for extra unicode functions
 script: ./autogen.sh && make && make check
 notifications:
   irc: "irc.freenode.org#harfbuzz"