Fix environment in grunt task
Fix environment in grunt task

When setting the `env` option on a grunt-exec task, this will *replace*
all environment variables rather than just adding the specified ones.
As a result, the `$PATH` is not available anymore and the `node`
executable is not found.

Manually merge BABEL_ENV with the current environment.

Change-Id: I9b1fb01693e4bc976679acbd6390b4fab345d580

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js