Travis: use addons to install libncursesw5

.travis.yml

@@ -37,6 +37,7 @@ addons:
   apt:
     packages:
     - ninja-build
+    - libncursesw5
 
 matrix:
   include:
@@ -138,7 +139,6 @@ matrix:
       name: "spellcheck"
       env: NAME=doxy-spellcheck
       install:
-        - sudo apt-get -y install libncursesw5
         - source_pkg aspell
       script:
         # TODO: run checks on all directories once all mispellings are fixed