Hide STDERR output when looking for proper browser executable.

[eNcacz]

When the library tries to found browser version, then it runs (for example) this command:

google-chrome --version || google-chrome-stable --version || google-chrome-beta --version || google-chrome-dev --version

When the first executable (the google-chrome in example above) is not found, then this error message is printed to STDERR:

/bin/sh: line 1: google-chrome: command not found

This pull request redirects STDERR of the above command to /dev/null so the message will not affect error output of the application.