travis: Add linux build script from citra

.travis/linux/build.sh

@@ -0,0 +1,3 @@
+#!/bin/bash -ex
+mkdir -p "$HOME/.ccache"
+docker run -v $(pwd):/threeSD -v "$HOME/.ccache":/root/.ccache citraemu/build-environments:linux-fresh /bin/bash -ex /threeSD/.travis/linux/docker.sh

.travis/linux/deps.sh

@@ -0,0 +1,3 @@
+#!/bin/sh -ex
+
+docker pull citraemu/build-environments:linux-fresh

.travis/linux/docker.sh

@@ -0,0 +1,9 @@
+#!/bin/bash -ex
+
+cd /citra
+
+mkdir build && cd build
+cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=/usr/lib/ccache/gcc -DCMAKE_CXX_COMPILER=/usr/lib/ccache/g++ -DENABLE_QT_TRANSLATION=ON -DCITRA_ENABLE_COMPATIBILITY_REPORTING=${ENABLE_COMPATIBILITY_REPORTING:-"OFF"} -DENABLE_COMPATIBILITY_LIST_DOWNLOAD=ON -DUSE_DISCORD_PRESENCE=ON -DENABLE_FFMPEG_VIDEO_DUMPER=ON
+ninja
+
+ctest -VV -C Release