Update Travis dist to xenial

.travis.yml

@@ -7,15 +7,15 @@ r:
 sudo: false
 cache: packages
 
-# dependency for gsl>copula>zinbwave
+dist: xenial
 addons:
   apt:
     packages:
-      - libgsl-dev
+      - libgsl-dev # dependency for gsl>copula>zinbwave
 
-install:
-  - R -e 'install.packages("devtools")'
-  - R -e 'devtools::install(dependencies = TRUE, upgrade_dependencies = TRUE)'
+#install:
+#  - R -e 'install.packages("devtools")'
+#  - R -e 'devtools::install(dependencies = TRUE, upgrade_dependencies = TRUE)'
 
 after_success:
   - Rscript -e 'covr::codecov()'