travis: Try fixing gcc build

.travis.yml

@@ -26,6 +26,10 @@ before_install:
   - sudo apt-get update -qq
   - sudo apt-get --force-yes install -qq libqt4-dev libsqlite3-dev libsqlcipher-dev libantlr-dev
 
+install:
+  - if [ "$CXX" = "g++" ]; then export CXX="g++-5" CC="gcc-5"; fi
+  - if [ "$CXX" = "clang++" ]; then export CXX="clang++-3.7" CC="clang-3.7"; fi
+
 script:
   - mkdir build
   - mkdir build_cipher