[ci] use the latest versions of OSes at Travis (#2467)

* bump images

* updated linkchecker's config

* test: switch compilers

* switch compilers back

.travis.yml

@@ -6,8 +6,8 @@ git:
 os:
   - linux
   - osx
-dist: trusty
-osx_image: xcode10.2
+dist: bionic
+osx_image: xcode11
 
 env:
   global:  # default values

docs/.linkcheckerrc

@@ -5,10 +5,9 @@ sslverify=0
 
 [filtering]
 ignore=
-  public.tableau.com
-  https://www.open-mpi.org
-  https://readthedocs.org
+  pythonapi/lightgbm\..*\.html.*
 ignorewarnings=http-robots-denied,https-certificate-error
+checkextern=1
 
 [output]
 # Set to 1 if you want see the full output, not only warnings and errors