Bump nginx to 1.7.5

nginx 1.7.5 is out, has a few bugfixes, etc.  Codebase changed enough
from 1.7.4 that the patch against src/http/ngx_http_upstream.[ch] no
longer applies (and wouldn't with a very smart and fuzz-tolerant patch
utility).  Oddly, whatever was in src/http/ngx_http_upstream.[ch] in
nginx <=1.7.4 no longer seems to be an issue for the ajp module
(compiles, runs without segfaulting, connects to Tomcat), so a stupid
"let's delete a bunch of lines from a set of modifications to source we
know nothing about" approach miraculously works.  Yaoweibin may have a
"proper" patch that still may fiddle with ngx_http_upstream.[ch] to fix
some non-obvious or minor issue in nginx 1.7.5's codebase, but I doubt
it.
	Hooray for blind luck!