more consistent use of environment variables

.ci/test_r_package.sh

@@ -70,7 +70,7 @@ Rscript -e "install.packages(c('data.table', 'jsonlite', 'Matrix', 'R6', 'testth
 cd ${BUILD_DIRECTORY}
 Rscript build_r.R || exit -1
 
-PKG_TARBALL=$(ls | grep '^lightgbm_.*\.tar\.gz$')
+PKG_TARBALL="lightgbm_${LGB_VER}.tar.gz"
 LOG_FILE_NAME="lightgbm.Rcheck/00check.log"
 
 # suppress R CMD check warning from Suggests dependencies not being available

.travis.yml

@@ -24,7 +24,7 @@ env:
     - TASK=mpi METHOD=pip
     - TASK=gpu METHOD=source PYTHON_VERSION=3.5
     - TASK=gpu METHOD=pip PYTHON_VERSION=3.6
-    - TASK=r-package R_MAC_VERSION=3.6.1 R_TRAVIS_LINUX_VERSION=3.6.1-3bionic
+    - TASK=r-package
 
 matrix:
   exclude:
@@ -45,9 +45,11 @@ before_install:
   - if [[ $TRAVIS_OS_NAME == "osx" ]]; then
         export OS_NAME="macos";
         export COMPILER="clang";
+        export R_MAC_VERSION=3.6.1;
     else
         export OS_NAME="linux";
         export COMPILER="gcc";
+        export R_TRAVIS_LINUX_VERSION=3.6.1-3bionic;
     fi
   - export CONDA="$HOME/miniconda"
   - export PATH="$CONDA/bin:$PATH"