Fix insecure 'curl' command in install instructions

Fixes two issues:

- curl was given -k aka --insecure, which causes it to ignore SSL certificate
  errors. This makes SSL nearly useless against active tampering.

- curl was given a URL with no protocol, causing it to default to unencrypted
  HTTP.  An active attacker can filter out GitHub's HTTPS redirect or modify
  the response in arbitrary other ways.

Either of these issues makes it quite easy for an active attacker, anywhere on
the network between GitHub and the person installing Homebrew, to insert
malicious code. Since this code goes straight into the ruby interpreter, this
is a one-step compromise of the user's machine.

Since the command got longer, shrink the font so that it still fits in the page
column (at maximum width) without scrolling.

On 10.5 the curl command can fail due to missing SSL certs.  Add a note to look
at the alternate install instructions on the wiki, which now mention curl
--insecure .