Utility function to get upgradable packages

    Sample return data:
    { 'pkgname': '1.2.3-45', ... }

def _get_upgradable(backtrack=3):
    '''
    Utility function to get upgradable packages

    Sample return data:
    { 'pkgname': '1.2.3-45', ... }
    '''

    cmd = ['emerge',
           '--ask', 'n',
           '--backtrack', '{0}'.format(backtrack),
           '--pretend',
           '--update',
           '--newuse',
           '--deep',
           '@world']

    call = __salt__['cmd.run_all'](cmd,
                                   output_loglevel='trace',
                                   python_shell=False)

    if call['retcode'] != 0:
        msg = 'Failed to get upgrades'
        for key in ('stderr', 'stdout'):
            if call[key]:
                msg += ': ' + call[key]
                break
        raise CommandExecutionError(msg)
    else:
        out = call['stdout']

    rexp = re.compile(r'(?m)^\[.+\] '
                      r'([^ ]+/[^ ]+)'    # Package string
                      '-'
                      r'([0-9]+[^ ]+)'          # Version
                      r'.*$')
    keys = ['name', 'version']
    _get = lambda l, k: l[keys.index(k)]

    upgrades = rexp.findall(out)

    ret = {}
    for line in upgrades:
        name = _get(line, 'name')
        version_num = _get(line, 'version')
        ret[name] = version_num

    return ret