use FontAwesome icon-envelope instead of icon-envelope-alt

When `email: test@email.com` line is added to `_config.rb`, the email icon placed in the navigation bar is `icon-envelope-alt`. This doesn't fit as well with the more solid-colored versions of the other (GitHub, LinkedIn, etc) icons.

The PR replaces `icon-envelope-alt` with `icon-envelope`.

Screenshots:
[Before](http://i.imgur.com/QKAPeve)
[After](http://i.imgur.com/0D5176r.png)