README.md: fix ubuntu package name

1) The package curl-dev does not exist. See
   http://packages.ubuntu.com/precise/curl-dev

2) The correct package libcurl-dev is a virtual package. Update the
   readme to default to the recommended working variant (libcurl4 with
   OpenSSL) so that it will "just work" for an inexperienced user. But,
   include enough hints that a very advanced user will recognize that
   she can do something different.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>