Project: Ruby in WebDev101 needs a warning about rspec version

[Wolfenswan]

It seems that the test-first gem-file has a bug in it's rspec requirement (see the Edit of the first answer here ) and thus won't work with rspec 3.

A warning and instructions on the course page on how to install an older version of rspec should be sufficient but before I open a PR I'd rather get confirmation on that.

Note:
I've also opened an issue (alexch/learn_ruby#23) on the tutorial's repository. So this issue might simply become superfluous.

[Wolfenswan]

Closing this issue as it was resolved via #530