Remove specific bundler version information, no longer needed

When retesting the minimum scenario today
https://github.com/ianfixes/arduino_ci/issues/69#issuecomment-457851016
I used just

    gem install bundler

and there were no problems like there had been earlier. So things have
apparently been resolved and the version specific install workaround is
no longer needed.

I tested using a separate user starting with an empty home directory,
e.g. guaranteed no old stale things being picked up.

Author: hlovdal

CONTRIBUTING.md

@@ -18,15 +18,11 @@ Pull requests will trigger a Travis CI job.  The following two commands will be
 apt-get install ruby ruby-dev    # For Debian/Ubuntu
 dnf install ruby ruby-devel      # For Fedora
 yum install ruby ruby-devel      # For Centos/RHEL
-gem install bundler              # See note below about version
+gem install bundler
 gem install rubocop
 gem install rspec
 ```
 
-As of writing this you want install a version 1 of bundler, `gem install bundler -v '1.17.3'`,
-since there is some incompability with regards to the
-[latest version 2 release of bundler](https://bundler.io/blog/2019/01/04/an-update-on-the-bundler-2-release.html)
-
 Be prepared to write tests to accompany any code you would like to see merged.
 See `SampleProjects/TestSomething/test/*.cpp` for the existing tests (run by rspec).