Turn off lintr-bot on travis-ci

we are currently using `lintr` in `testthat` tests -- this is apparently not intended usage (see https://github.com/jimhester/lintr#continuous-integration) and thus our setup does not respect `.lintr` settings (r-lib/lintr#345)

--> keep our current setup and turn of verbose `lintr-bot`

.travis.yml

@@ -1,15 +1,24 @@
 # R for travis: see documentation at https://docs.travis-ci.com/user/languages/r
 
 language: r
+
 r:
   - oldrel
   - release
   - devel
+
+env:
+  global:
+    - LINTR_COMMENT_BOT=false
+
 sudo: false
+
 warnings_are_errors: false # set to true: This option forces all WARNINGS from R CMD check
   # to become build failures (default true). This is especially helpful when preparing
   # your package for submission to CRAN
+
 cache: packages
+
 after_success:
   # code coverage but exclude all SOILWAT2 submodule files in scr/
   - Rscript -e 'covr::coveralls(line_exclusions = as.list(file.path("src", list.files("src", recursive = TRUE))))'