To match and encapsulate from rpm.

.rpm packages can now be build with:

    gulp vscode-linux-<arch>-build-rpm

The resulting package will be output to ~/rpmbuild/RPMS/<arch>/ and can be
installed on a Red Hat-based distro with:

    yum localinstall <path>

Note that Debian systems can build the package after installing the 'rpm'
package:

    apt-get install rpm

This was tested building 64-bit on Ubuntu 15.10 and installing on CentOS 7.

Fixes #3595