minimum curl version effectively changed

As of ad9bb6dfe6e598d87ffe6e2285b4b86dac3bc726, http.c depends on symbols introduced curl 7.37.0, which is newer than the documented minimum version of 7.21.3 in INSTALL.

```
In file included from /usr/include/curl/curl.h:2238:0,
                 from git-curl-compat.h:3,
                 from http.c:4:
http.c: In function ‘set_proxyauth_name_password’:
http.c:655:28: error: ‘CURLOPT_PROXYHEADER’ undeclared (first use in
this function)
   curl_easy_setopt(result, CURLOPT_PROXYHEADER,
                            ^
http.c:655:28: note: each undeclared identifier is reported only once
for each function it appears in
make: *** [http.o] Error 1
```

-Alejandro