TEMPORARY: don't build vignettes

current version of `RandomFields` v3.3.1 hangs and prevents vignette building. The yet-to-be-released v3.3.4 fixes the issue, so wait until that's available to restore vignette building on travis

.travis.yml

@@ -25,6 +25,8 @@ matrix:
       - "export DISPLAY=:99.0"
       - "sh -e /etc/init.d/xvfb start"
       - sleep 3 # give xvfb some time to start
+    r_build_args: "--no-build-vignettes"
+    r_check_args: "--as-cran --ignore-vignettes --no-examples"
   - os: linux
     dist: trusty
     r: 3.4
@@ -43,6 +45,8 @@ matrix:
       - "export DISPLAY=:99.0"
       - "sh -e /etc/init.d/xvfb start"
       - sleep 3 # give xvfb some time to start
+    r_build_args: "--no-build-vignettes"
+    r_check_args: "--as-cran --ignore-vignettes --no-examples"
   - os: linux
     dist: trusty
     r: oldrel
@@ -61,6 +65,8 @@ matrix:
       - "export DISPLAY=:99.0"
       - "sh -e /etc/init.d/xvfb start"
       - sleep 3 # give xvfb some time to start
+    r_build_args: "--no-build-vignettes"
+    r_check_args: "--as-cran --ignore-vignettes --no-examples"
   - os: linux
     dist: trusty
     r: release
@@ -85,6 +91,8 @@ matrix:
     after_success:
       - travis_wait 20 Rscript -e 'covr::coveralls(type = "all", quiet = FALSE)'
       - Rscript -e 'lintr::lint_package()'
+    r_build_args: "--no-build-vignettes"
+    r_check_args: "--as-cran --ignore-vignettes --no-examples"
     r_github_packages:
       - jimhester/lintr
       - r-lib/covr
@@ -108,6 +116,8 @@ matrix:
       - "export DISPLAY=:99.0"
       - "sh -e /etc/init.d/xvfb start"
       - sleep 3 # give xvfb some time to start
+    r_build_args: "--no-build-vignettes"
+    r_check_args: "--as-cran --ignore-vignettes --no-examples"
     r_packages:
       - devtools
   - os: osx