travis: Install libgnutls-dev prior to building TW

Author: tbabej

.travis.yml

@@ -23,7 +23,7 @@ install:
   - pip install coveralls
   - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
   - sudo apt-get update -qq
-  - sudo apt-get install -qq build-essential cmake uuid-dev g++-4.8
+  - sudo apt-get install -qq build-essential cmake uuid-dev libgnutls-dev g++-4.8
   - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 50
   - git clone --recursive https://github.com/GothenburgBitFactory/taskwarrior
   - cd taskwarrior