AVX gradients (#2878)

* AVX gradients

* Add google test for AVX

* Create fallback implementation, remove fma instruction

* Improved accuracy of AVX exp function

tests/travis/run_test.sh

@@ -91,10 +91,30 @@ if [ ${TASK} == "java_test" ]; then
 fi
 
 if [ ${TASK} == "cmake_test" ]; then
-    mkdir build
-    cd build
-    cmake ..
+    set -e
+    # Build gtest via cmake
+    wget https://github.com/google/googletest/archive/release-1.7.0.zip
+    unzip release-1.7.0.zip
+    mv googletest-release-1.7.0 gtest && cd gtest
+    cmake . && make
+    mkdir lib && mv libgtest.a lib
+    cd ..
+    rm -rf release-1.7.0.zip
+
+    # Build/test without AVX
+    mkdir build && cd build
+    cmake .. -DGOOGLE_TEST=ON 
     make
+    cd ..
+    ./testxgboost
+    rm -rf build
+    
+    # Build/test with AVX
+    mkdir build && cd build
+    cmake .. -DGOOGLE_TEST=ON -DUSE_AVX=ON
+    make
+    cd ..
+    ./testxgboost
 fi
 
 if [ ${TASK} == "cpp_test" ]; then