[docs] fix RTD builds due to conda's problems (#2725)

* fix RTD builds

* more pins

* hotfix

* removed unnecessary pinning

docs/conf.py

@@ -231,9 +231,10 @@ def generate_r_docs(app):
         r-jsonlite=1.5=r351h96ca727_0 \
         r-matrix=1.2_14=r351h96ca727_0 \
         r-testthat=2.0.0=r351h29659fb_0 \
-        cmake=3.14.0=h52cb24c_0
+        cmake=3.14.0=h52cb24c_0 \
+        ca-certificates=2019.11.27=0
     /home/docs/.conda/bin/conda install -q -y -n r_env -c conda-forge \
-        r-pkgdown=1.3.0=r351h6115d3f_1000 \
+        r-pkgdown=1.3.0=r35h6115d3f_1001 \
         r-roxygen2=6.1.1=r35h0357c0b_1001
     source /home/docs/.conda/bin/activate r_env
     export TAR=/bin/tar