http2: spell fixed README and added version requirement
tests: add test for bug #1303 (dns cache timeout)

Test-case 1515 reproduces bug #1303, where libcurl
would incorrectly prune DNS entries added via
CURLOPT_RESOLVE after the DNS_CACHE_TIMEOUT had
expired.
tests: add test for bug #1327 (dns cache timeout)

Fix for bug #1303 (030a2b8cb) was not complete.
libcurl still pruned DNS entries added manually
after detecting a dead connection. This test
checks such behavior.