chore: update TravisCI OSx minimum supported version

The reason to update is, that brew only supports 10.12 officialy and
with binary packages so our CI builds started failing because compiling took too long.

.travis.yml

@@ -105,7 +105,7 @@ matrix:
           - /opt/build-windows/x86_64
     - stage: "macOS, AppImage and Flatpak"
       os: osx
-      osx_image: xcode7.3
+      osx_image: xcode9.2
       env: JOB=build-osx
     - stage: "macOS, AppImage and Flatpak"
       os: linux

.travis/build-osx.sh

@@ -17,7 +17,7 @@
 #
 
 # Fail out on error
-set -e -o pipefail
+set -eu -o pipefail
 
 # accelerate builds with ccache
 install_ccache() {
@@ -43,16 +43,7 @@ check() {
     fi
 }
 
-# The system ruby in Travis CI is too old, use latest stable
-get_ruby_version() {
-    rvm get stable
-    [[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm"
-    rvm use ruby --install --default
-    echo rvm_auto_reload_flag=1 >> ~/.rvmrc
-}
-
 main() {
-    get_ruby_version
     install_ccache
     build
     check

INSTALL.md

@@ -579,11 +579,10 @@ password for sudo.
 
 ## OS X
 
-Supported OS X versions: >=10.8.
+Supported OS X versions: >=10.8. (NOTE: only 10.12 is tested during CI)
 
-Compiling qTox on OS X for development requires 3 tools:
+Compiling qTox on OS X for development requires 2 tools:
-[Xcode](https://developer.apple.com/xcode/),
+[Xcode](https://developer.apple.com/xcode/) and [homebrew](https://brew.sh).
-[Qt 5.4+](https://www.qt.io/qt5-4/) and [homebrew](https://brew.sh).
 
 ### Automated Script