Fix thread-unsafe lazy initialization

[balexand]

The lazy initialization of env was not thread-safe. See #11 (comment).

Note that this gem is still not thread-safe when there is a chance that ENV will be accessed by other threads in an application. See #11.

[jferris]

This looks good to me. @joshuaclayton?

[joshuaclayton]

@balexand @jferris yep, looks good; merged as 7405df9