coverity: use https URL for posting build

When posting our instrumented build results to Coverity we have
to include sensitive information, in particular our authorization
token. Currently we use an unencrypted channel to post this
information, leading to the token being transferred in plain.

Fix this by using a secured connection instead.

script/coverity.sh

@@ -50,10 +50,9 @@ COVERITY_UNSUPPORTED=1 \
 tar czf libgit2.tgz cov-int
 SHA=$(git rev-parse --short HEAD)
 curl \
-	--form project=libgit2 \
 	--form token="$COVERITY_TOKEN" \
 	--form email=bs@github.com \
 	--form file=@libgit2.tgz \
 	--form version="$SHA" \
 	--form description="Travis build" \
-	http://scan5.coverity.com/cgi-bin/upload.py
+	https://scan.coverity.com/builds?project=libgit2