Only set up default locale on FixMyStreet install.

FixMyStreet uses the default locale in order to have working URIs with
floating point numbers. This could be moved to the FixMyStreet install
script with its next release and then removed from here.
My understanding (which could well be wrong!) is that you need to make the locales _available_ through `dpkg-reconfigure locales`. Basically this step:

![screen shot 2014-07-18 at 10 49 49](https://cloud.githubusercontent.com/assets/282788/3624727/e76f491a-0e60-11e4-939b-de351e5a22da.png)