Adjust supported Ruby versions

Ruby 2.4 is now end-of-life. As a result of this, the `google-protobuf`
gem is no longer releasing artifacts compatible with this version.
(protocolbuffers/protobuf#7453)

In CI we encounter this error from `bundle install`:
```
google-protobuf-3.12.0-x86_64-linux requires ruby version >= 2.5, which is
incompatible with the current version, ruby 2.4.6p354
```

Therefore remove Ruby 2.4.6 as a tested version in Travis CI.

Additionally remove the constraint on patch versions in the Travis
config, so that we'll use the latest patch version available for each
release branch.

.travis.yml

@@ -1,9 +1,9 @@
 language: ruby
 
 rvm:
-  - 2.4.6
-  - 2.5.5
-  - 2.6.3
+  - 2.5
+  - 2.6
+  - 2.7
   - ruby-head
 
 gemfile: