Travis: Update Homebrew/fix Mac CI build

Updating Homebrew wasn't necessary when the CI build first started using
the xcode8.3 image (see c8e52741fdb623698a27a4c55cf63db93c95aa14), but
apparently it now is.
diff --git a/.travis.yml b/.travis.yml
index 94e7589..04eff2c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -55,6 +55,7 @@
 addons:
   homebrew:
     brewfile: true
+    update: true
 
 before_install:
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then