travis: Install realpath, required for teensy build.

.travis.yml

@@ -8,6 +8,8 @@ before_script:
   - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
   - sudo apt-get update -qq
   - sudo apt-get install -y python3.3 python3 gcc-4.7 gcc-arm-none-eabi qemu-system mingw32
+  # For teensy build
+  - sudo apt-get install realpath
 
 script:
   - make -C unix CC=gcc-4.7