Adds Ruby 2.5 and 2.6 to Travis CI configuration (#22)

* Adds Ruby 2.5.5 and 2.6.3 to Travis CI configuration

Adds test coverage for newer versions of Ruby. Updates Ruby 2.3, 2.4,
and 2.5 patch version to the latest available.

* Remove patch version from Travis CI configuration

When you omit the patch version from the rvm configuration, Travis CI
will use the latest patch version available for that minor version of
Ruby.  i.e. specify version 2.6 will download 2.6.3 as of this commit.

Author: scottserok

.travis.yml

@@ -1,6 +1,7 @@
 language: ruby
 rvm:
-  - 2.3.4
-  - 2.4.1
+  - 2.4
+  - 2.5
+  - 2.6
   - ruby-head
 before_install: gem update bundler