Add dev.sh to avoid setting GEM_HOME and GEM_PATH

[k0kubun]

Closes #422

I added dev.sh file as per #422 (comment).

[postmodern]

Spotted a bug on line 59. I also do not think dev.sh is necessary if it only sets a global variable.

Also, I am hesitant about adding more global variables. I think hook functions might be a better option. I may try to make a separate branch of the 0.4.0 branch to demonstrate the idea of extracting the GEM_HOME/GEM_PATH logic into a hook function that can be overrode.

[postmodern]

Now this obviously needs tests. The unit tests are located in the test/unit/ directory and use shunit2 which you should be able to install via your system's package manager (brew install shunit2 or sudo apt install -y shunit2). You can probably copy the existing tests from test/unit/chruby_use_test.sh but test that GEM_HOME and GEM_PATH do not get set. The test files can be ran locally by simply executing the file, since the *_test.s files are just shell scripts.

[k0kubun]

Thanks for the advice. I copied test/unit/chruby_use_test.sh to test/unit/chruby_dev_test.sh and modified it for the changed behavior. I'd appreciate another look when you have a chance.

[postmodern]

The tests all passed! I will have to manually merge this into the 0.4.0 branch. Then I will have to start working on release engineering and preparing a 0.4.0 release. I may also need to do a 0.3.9 release since the main branch contains a fix for detecting mruby ruby versions.

[postmodern]

Manually merged into 0.4.0.