Improve irb_spec's home setup

[st0012]

Before v1.12, IRB avoids loading ~/.irbrc if the project folder has a .irbrc file. So to avoid loading dev's ~/.irbrc, cfe908c added an empty irbrc fixture as a workaround.

However, because IRB v1.12 starts loading ~/.irbrc even if the project folder has a .irbrc file, the workaround no longer works.

This commit removes the workaround and uses altering HOME environment variable to avoid loading .irbrc from devs' home directory instead.

[nobu]

Don't IRBRC and XDG_CONFIG_HOME need to be set or cleared too?

[eregon]

Could we use irb's -f flag or similar to skip irbrc's?