Upgrade `wagon-http` to version 2.6

There is a bug in Apache Maven Wagon v2.4 that manifests when I attempt
to run `lein` on a machine behind our firewall; I get the following
error multiple times, then the build fails:

```
Oct 03, 2014 8:24:02 PM org.apache.http.impl.client.DefaultHttpClient tryConnect
INFO: I/O exception (java.net.SocketException) caught when connecting to {s}->https://clojars.org: Connection reset
```

There seems to be something wrong with the way Wagon 2.4 uses
HttpClient (likely because Wagon 2.4 uses a much older verion of
HttpClient as well as a now-deprecated mechanism for creating the
HttpClient instance.)

Simply upgrading to version 2.6 and rebuilding Leiningen
solves the problem.