diff --git a/.ci/setup.sh b/.ci/setup.sh
index 18af055a53b..8bfbc714136 100755
--- a/.ci/setup.sh
+++ b/.ci/setup.sh
@@ -22,21 +22,25 @@ if [[ $OS_NAME == "macos" ]]; then
         wget -O conda.sh https://repo.continuum.io/miniconda/Miniconda${PYTHON_VERSION:0:1}-latest-MacOSX-x86_64.sh
     fi
 else  # Linux
+    sudo apt-get update
     if [[ $AZURE == "true" ]] && [[ $COMPILER == "clang" ]]; then
-        update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-6.0 100
-        update-alternatives --install /usr/bin/clang clang /usr/bin/clang-6.0 100
-        sudo apt-get update
+        sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-6.0 100
+        sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-6.0 100
         sudo apt-get install libomp-dev
+    elif [[ $AZURE == "true" ]] && [[ $COMPILER == "gcc" ]] && [[ $TASK != "gpu" ]]; then
+        # downgrade gcc version
+        sudo apt-get remove -y gcc || exit -1
+        sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+        sudo apt-get update
+        sudo apt-get install --no-install-recommends -y g++-4.8 || exit -1
+        sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 100
+        sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 100
     fi
     if [[ $TASK == "mpi" ]]; then
-        if [[ $AZURE == "true" ]]; then
-            sudo apt-get update
-        fi
-        sudo apt-get install -y libopenmpi-dev openmpi-bin
+        sudo apt-get install --no-install-recommends -y libopenmpi-dev openmpi-bin
     fi
     if [[ $TASK == "gpu" ]]; then
         if [[ $AZURE == "true" ]]; then
-            sudo apt-get update
             sudo apt-get install --no-install-recommends -y libboost-dev libboost-system-dev libboost-filesystem-dev
         fi
         sudo apt-get install --no-install-recommends -y ocl-icd-opencl-dev
diff --git a/.vsts-ci.yml b/.vsts-ci.yml
index 24679d26e7c..ef29a30005b 100644
--- a/.vsts-ci.yml
+++ b/.vsts-ci.yml
@@ -53,7 +53,7 @@ phases:
   - bash: $(Build.SourcesDirectory)/.ci/test.sh
     displayName: Test
   - task: PublishBuildArtifacts@1
-    condition: and(succeeded(), startsWith(variables['Build.SourceBranch'], 'refs/tags/'))
+    condition: and(succeeded(), not(startsWith(variables['Build.SourceBranch'], 'refs/pull/')))
     inputs:
       pathtoPublish: '$(Build.ArtifactStagingDirectory)'
       artifactName: PackageAssets
@@ -64,7 +64,7 @@ phases:
   variables:
     COMPILER: clang
   queue:
-    name: 'Hosted macOS Preview'
+    name: 'Hosted macOS'
     parallel: 3
     matrix:
       regular:
@@ -97,7 +97,7 @@ phases:
   - bash: $(Build.SourcesDirectory)/.ci/test.sh
     displayName: Test
   - task: PublishBuildArtifacts@1
-    condition: and(succeeded(), startsWith(variables['Build.SourceBranch'], 'refs/tags/'))
+    condition: and(succeeded(), not(startsWith(variables['Build.SourceBranch'], 'refs/pull/')))
     inputs:
       pathtoPublish: '$(Build.ArtifactStagingDirectory)' 
       artifactName: PackageAssets
@@ -128,7 +128,7 @@ phases:
   - powershell: $(Build.SourcesDirectory)/.ci/test_windows.ps1
     displayName: Test
   - task: PublishBuildArtifacts@1
-    condition: and(succeeded(), startsWith(variables['Build.SourceBranch'], 'refs/tags/'))
+    condition: and(succeeded(), not(startsWith(variables['Build.SourceBranch'], 'refs/pull/')))
     inputs:
       pathtoPublish: '$(Build.ArtifactStagingDirectory)'
       artifactName: PackageAssets
@@ -141,7 +141,7 @@ phases:
   - Linux
   - MacOS
   - Windows
-  condition: and(succeeded(), startsWith(variables['Build.SourceBranch'], 'refs/tags/'))
+  condition: and(succeeded(), not(startsWith(variables['Build.SourceBranch'], 'refs/pull/')))
   queue:
     name: 'Hosted VS2017'
   steps:
diff --git a/src/metric/regression_metric.hpp b/src/metric/regression_metric.hpp
index 252ce7c1390..5007fbd76fa 100644
--- a/src/metric/regression_metric.hpp
+++ b/src/metric/regression_metric.hpp
@@ -252,7 +252,7 @@ class GammaMetric : public RegressionMetric<GammaMetric> {
     const double theta = -1.0 / score;
     const double a = psi;
     const double b = -std::log(-theta);
-    const double c = 1. / psi * std::log(label / psi) - std::log(label) - std::lgamma(1.0 / psi);
+    const double c = 1. / psi * std::log(label / psi) - std::log(label) - 0; // 0 = std::lgamma(1.0 / psi) = std::lgamma(1.0);
     return -((label * theta - b) / a + c);
   }
   inline static const char* Name() {