INSTALL: depend on URI rather than URI::Escape

INSTALL: depend on URI rather than URI::Escape

As far as I can tell, URI::Escape has always been a part of the
`URI' package (aka "distribution" on CPAN) and not distributed
separately (unlike URI::Escape::XS).  So avoid confusing users
with `URI::Escape' and just document `URI' instead.

Along the same lines, we depend on the `Plack' package rather
than Plack::Util or Plack::Builder, after all.