Adds check for asdf presence

[idabmat]

Hi! Thank you for this gem.

Adding another lovely version manager to the mix: asdf

Unfortunately I was not able to run the test suite on my machine. Making a PR and delegating that job to Travis 🤞

[grosser]

idk if the other executable are tested, but if they are can you add a test for asdf too ?

[grosser]

looks about right
... what error do you see when running tests ?

[idabmat]

It took me a while to setup a Docker image that included rvm and the different ruby version. (Vagrant was not an option for me).

The tests just finished running on my machine. I get 2 failures:

• jruby-9.2 was not installed
• WWTD CLI --parallel runs in parallel because my machine took too long to run the test

[grosser]

I was hoping most of the tests would just work if there are different ruby versions installed locally :(

[idabmat]

🤦‍♂️ oh you're right! I didn't think of that. I'll install the different versions and run the test suite locally and report back.