travis.yml: Install gcc via homebrew addon

Related to https://github.com/JuulLabs-OSS/mynewt-travis-ci/pull/92
diff --git a/.travis.yml b/.travis.yml
index 364de3a..6d692ce 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -128,18 +128,30 @@
         - TEST=TEST_ALL
         - VM_AMOUNT=3
         - TARGET_SET=1
+      addons:
+        homebrew:
+          packages:
+          - gcc@5
     - os: osx
       osx_image: xcode9.2
       env:
         - TEST=TEST_ALL
         - VM_AMOUNT=3
         - TARGET_SET=2
+      addons:
+        homebrew:
+          packages:
+          - gcc@5
     - os: osx
       osx_image: xcode9.2
       env:
         - TEST=TEST_ALL
         - VM_AMOUNT=3
         - TARGET_SET=3
+      addons:
+        homebrew:
+          packages:
+          - gcc@5
     - os: windows
       env:
         - TEST=BUILD_TARGETS_WINDOWS