update travis yml

trusty is now the default
move to travis homebrew addon

.travis.yml

@@ -1,6 +1,5 @@
 language: cpp
 
-dist: trusty
 sudo: false
 
 os:
@@ -13,26 +12,12 @@ compiler:
   - clang
   - gcc
 
-before_install:
-  - if [ $TRAVIS_OS_NAME = osx ]; then
-      brew update;
-    fi;
-
-install:
-  - if [ $TRAVIS_OS_NAME = linux ]; then
-      if [ "$CXX" = "g++" ]; then
-        export CXX=g++-7;
-      fi;
-    elif [ $TRAVIS_OS_NAME = osx ]; then
-      if [ "$CXX" = "g++" ]; then
-        brew install gcc@7;
-        export CXX=g++-7;
-      fi;
-    fi;
 env:
   global:
    - secure: "R+NxqtytOslIcQ/eCbLoZhImsgYdJnljfjANdieFQGune9ACPPQL0YanXkF49c9SWGBSxrAcute0egQzv2CU2+ivSQIX/xnMebKHiOmSPYBoxX+VgxLT3U1itUYlpYwixo9rF8UnGdlgXid6oENSiCvfWtNKoM2qOL0Ttw31J9E="
 
+before_install: if [ "$CXX" = "g++" ]; then export CXX=g++-7; fi
+
 addons:
   apt:
     sources:
@@ -41,6 +26,9 @@ addons:
       - libstdc++-7-dev
       - g++-7
       - libncursesw5-dev
+  homebrew:
+    update: true
+    packages: gcc@7
   coverity_scan:
     project:
       name: "mawww/kakoune"