Drop undefined/null values from child process env

Undefined LANG was causing an actual LANG="undefined" to be passed
to the child process.
Set LANG in the upstart/init script on Ubuntu.
Merge pull request #72 from rstudio/bugfix/lang-undefined

Drop undefined/null values from child process env
Merge pull request #73 from rstudio/feature/set-lang

Set LANG in the upstart/init script on Ubuntu.