travis: Split build scripts for different platforms

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

.travis/linux/docker.sh

@@ -0,0 +1,17 @@
+#!/bin/bash -ex
+
+cd /citra
+
+apt-get update
+apt-get install -y build-essential libsdl2-dev qtbase5-dev libqt5opengl5-dev libcurl4-openssl-dev libssl-dev wget git
+
+# Get a recent version of CMake
+wget https://cmake.org/files/v3.9/cmake-3.9.0-Linux-x86_64.sh
+echo y | sh cmake-3.9.0-Linux-x86_64.sh --prefix=cmake
+export PATH=/citra/cmake/cmake-3.9.0-Linux-x86_64/bin:$PATH
+
+mkdir build && cd build
+cmake .. -DUSE_SYSTEM_CURL=ON -DCMAKE_BUILD_TYPE=Release
+make -j4
+
+ctest -VV -C Release