travis: Split build scripts for different platforms

This commit also separates clang-format from the linux build, closing #2702.

.travis/macos/build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash -ex
+
+set -o pipefail
+
+export MACOSX_DEPLOYMENT_TARGET=10.9
+export Qt5_DIR=$(brew --prefix)/opt/qt5
+
+mkdir build && cd build
+cmake .. -DUSE_SYSTEM_CURL=ON -DCMAKE_OSX_ARCHITECTURES="x86_64;x86_64h" -DCMAKE_BUILD_TYPE=Release
+make -j4
+
+ctest -VV -C Release