Add troubleshooting to docs for higher Ruby versions

.gitignore

@@ -6,3 +6,4 @@ doc/
 vendor/
 profile/out/
 coverage/
+.ruby-version

CONTRIBUTING.md

@@ -22,12 +22,25 @@ And run the test suite (should be all green with pending tests):
 
 If you have problems installing nmatrix, please consult the [nmatrix installation wiki](https://github.com/SciRuby/nmatrix/wiki/Installation) or the [mailing list](https://groups.google.com/forum/#!forum/sciruby-dev).
 
+**NOTE**: We use Travis CI for automatic testing for Daru. Since we have 
+configured Travis CI only for Ruby versions 2.2, 2.3, 2.4, the test suite 
+returns the following error when `bundle exec rspec` is run for Ruby 
+version >= 2.5.
+
+```
+bundler: failed to load command: rspec (/home/.rbenv/versions/2.5.1/bin/rspec)
+...: command not found
+```
+To reproduce this issue or explore this error further, head over to 
+[issue #503](https://github.com/SciRuby/daru/issues/503).
+
 While preparing your pull requests, don't forget to check your code with Rubocop:
 
   `bundle exec rubocop`
 
 [Optional] Install all Ruby versions which Daru currently supports with `rake spec setup`.
 
+
 ## Basic Development Flow
 
 1. Create a new branch with `git checkout -b <branch_name>`.