[ci] upgrade to R 3.6.3 in CI (#2969)

.travis.yml

@@ -45,11 +45,11 @@ before_install:
   - if [[ $TRAVIS_OS_NAME == "osx" ]]; then
         export OS_NAME="macos";
         export COMPILER="gcc";
-        export R_MAC_VERSION=3.6.1;
+        export R_MAC_VERSION=3.6.3;
     else
         export OS_NAME="linux";
         export COMPILER="clang";
-        export R_TRAVIS_LINUX_VERSION=3.6.1-3bionic;
+        export R_TRAVIS_LINUX_VERSION=3.6.3-1bionic;
     fi
   - export CONDA="$HOME/miniconda"
   - export PATH="$CONDA/bin:$PATH"

.vsts-ci.yml

@@ -99,7 +99,7 @@ jobs:
       echo "##vso[task.setvariable variable=CONDA]$CONDA"
       echo "##vso[task.prependpath]$CONDA/bin"
       echo "##vso[task.setvariable variable=JAVA_HOME]$JAVA_HOME_8_X64"
-      echo "##vso[task.setvariable variable=R_MAC_VERSION]3.6.1"
+      echo "##vso[task.setvariable variable=R_MAC_VERSION]3.6.3"
     displayName: 'Set variables'
   - bash: $(Build.SourcesDirectory)/.ci/setup.sh
     displayName: Setup