README should indicate required bash version >=4 for tests to run

[mxsdev]

Had some issues for a while running tests locally on my new laptop because macOS keeps bash on version 3.2. Bash can't execute the prompt ${PS1@P} before (I believe) version 4, so kernel/session_test/testGetBash will fail, but it's really hard to debug the errors to realize that this is because of the bash version.

Part of #67

[adambabik]

There is https://github.com/stateful/runme/issues/105 which I filled in with description.