Update .travis.yml (#188)

Build server is obviously upgraded to Ubuntu 16.04, travis build is failing because of outdated qt591 repository.

.travis.yml

@@ -9,7 +9,7 @@ matrix:
           packages: ['clang-3.7', 'g++-8']
 
 before_install:
- - sudo add-apt-repository ppa:beineri/opt-qt591-trusty -y
+ - sudo add-apt-repository ppa:beineri/opt-qt591-xenial -y
  - sudo apt-get update -qq
  - sudo apt-get install qt59base qt59websockets 
  - source /opt/qt59/bin/qt59-env.sh
@@ -24,4 +24,4 @@ script:
  - qmake zec-qt-wallet.pro CONFIG+=release -spec linux-g++
  - res/libsodium/buildlibsodium.sh
  - make CC=gcc-8 CXX=g++-8 -j2
- 
+