WebService: Set USE_SYSTEM_CURL for travis linux builds

.travis-build-docker.sh

@@ -14,7 +14,7 @@ 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 .. -DCMAKE_BUILD_TYPE=Release
+cmake .. -DUSE_SYSTEM_CURL=ON -DCMAKE_BUILD_TYPE=Release
 make -j4
 
 ctest -VV -C Release