Update .travis.yml to remove 1.8.7 and REE

See #58

.travis.yml

@@ -15,8 +15,6 @@ rvm:
   - 2.2
   - 2.1
   - 1.9.3
-  - 1.8.7	
-  - ree
   # JRuby
   - jruby-head
   - jruby-9.2
@@ -29,15 +27,5 @@ jobs:
     - rvm: ruby-head
     - rvm: jruby-head
     - rvm: truffleruby-head
-    # NOTE (2020-12-18): Failing access to Rubygems with following error:
-    #   Unable to download data from https://rubygems.org/ - hostname was not
-    #   match with the server certificate (https://rubygems.org/specs.4.8.gz)
-    #
-    #   See Travis CI topic here:
-    #   https://travis-ci.community/t/ruby-1-8-7-and-ree-builds-broken-by-ssl-certificate-failure/10866
-    #
-    # TODO (2020-12-18): Either find a fix or remove testing of 1.8 versions
-    - rvm: 1.8.7
-    - rvm: ree
 
   fast_finish: true