workaround for travis problem

WARNING: The following packages cannot be authenticated!
  swig3.0
E: There are problems and -y was used without --force-yes

.travis.yml

@@ -8,7 +8,7 @@ before_install:
   - sudo add-apt-repository ppa:teward/swig3.0 -y
   - sudo apt-get update -qq
   - sudo apt-get install -y libboost-dev liblua5.1-0-dev lua5.1
-  - sudo apt-get install -y swig3.0
+  - sudo apt-get install -y --force-yes swig3.0
   - sudo apt-get install -y libxy-dev wxgtk-for-fityk-devel
 before_script:
   -  curl -o tests/catch.hpp https://raw.githubusercontent.com/philsquared/Catch/master/single_include/catch.hpp