Use nghttp2 from git for testing.

We'll want to return to a pinned version once a well-defined HPACK-9
release is shipped, but I'm sick of having my tests labelled 'broken'.

Author: Lukasa

.travis/install.sh

@@ -20,9 +20,8 @@ if [[ "$NGHTTP2" = true ]]; then
     pip install cython
 
     # Now, download and install nghttp2's latest version.
-    wget https://github.com/tatsuhiro-t/nghttp2/releases/download/v0.5.0/nghttp2-0.5.0.tar.gz
-    tar -xzvf nghttp2-0.5.0.tar.gz
-    cd nghttp2-0.5.0
+    git clone https://github.com/tatsuhiro-t/nghttp2.git
+    cd nghttp2
     autoreconf -i
     automake
     autoconf