Rename Arabic locale from ar_SA to ar to enable fallbacks

[hugovk]

Fixes #25.

Changes proposed in this pull request:

• Rename ar_SA to ar so it can be activated with both humanize.i18n.activate("ar_SA") and humanize.i18n.activate("ar_SA")
• Some other files don't need this, they already have fallbacks
• For example, this didn't work before, and does afterwards:

import humanize

print(humanize.ordinal(5))

for code in (
    "pt_BR",
    "pt_PT",
    "pt",
    "zh_CN",
    "zh_HK",
    "zh",
    "ar_SA",
    "ar",  # FileNotFoundError: [Errno 2] No translation file found for domain: 'humanize'
):
    print(code)
    x = humanize.i18n.activate(code)
    print(x.info())
    print(humanize.ordinal(5))
    humanize.i18n.deactivate()
    print()