fix(ci): upgrade travis to xcode9.3 to get macos 10.13 with brew support

Brew now only officially supports Mac OS 10.13-10.15, which means our
previously used 10.12 had fewer binary packages available, meaning we had to
compile more ourselves, causing CI to timeout.
https://docs.travis-ci.com/user/reference/osx#macos-version
https://github.com/Homebrew/brew/pull/6500

Fix #5515
  
An error occurred
 

.travis.yml

@@ -146,7 +146,7 @@ jobs:
           - /opt/build-windows/x86_64
     - stage: "macOS, AppImage and Flatpak"
       os: osx
-      osx_image: xcode9.2
+      osx_image: xcode9.3
       env: JOB=build-osx
       cache:
         timeout: 300 # seconds

INSTALL.md

@@ -597,7 +597,7 @@ See [AppArmor] to enable confinement for increased security.
 
 ## OS X
 
-Supported OS X versions: >=10.8. (NOTE: only 10.12 is tested during CI)
+Supported OS X versions: >=10.8. (NOTE: only 10.13 is tested during CI)
 
 Compiling qTox on OS X for development requires 2 tools:
 [Xcode](https://developer.apple.com/xcode/) and [homebrew](https://brew.sh).