Ensure that Yum configuration includes the (default) port

Yum requires that the proxy URI includes the port number. But
`URI::Generic#to_s` drops the port if it's the default for the specified
scheme, e.g. `80` for HTTP URIs.

So make sure that the port is always included. Also drop the path
(normally just a slash) as HTTP proxy URIs really don't use use.

Thanks to @partyzan and @johnbellone for all the information and help!

Fixes #36.
Release v1.0.1
Up version for development
Add Coveralls status
Add setting for disabling the plugin

The plugin can be totally skipped by setting `config.proxy.enabled` to
`false` or empty string (`""`). This can be useful to for example
disable it for some provider.
Add Ruby 2.0.0 to Travis's build matrix

Vagrant 1.4 will ship with Ruby 2.0, so let's run unit tests against it,
too.
Merge pull request #39 from tmatilai/config_enabled

Add setting for disabling the plugin
Update the change log
Update Vagrant to 1.4.0 for testing
Add support for Git proxy through system config.

This adds support for overriding the HTTP proxy for Git by setting the
system configuration. It uses the git command itself to write to the
config file.