Drop HTTP/2, as it doesn't work in Node 10 any more

.travis.yml

@@ -14,9 +14,4 @@ matrix:
   include:
     - node_js: "9"
       env: BROWSER=1
-    - node_js: "10"
-      env: HTTP2_TEST=1
 
-before_install:
-  - "test -z $(echo $HTTP2_TEST) || npm install --only=dev https://github.com/sogaani/express.git#initial-support-http2"
-  - "rm -f package-lock.json npm-shrinkwrap.json"

docs/index.md

@@ -73,15 +73,6 @@ The HTTP method defaults to __GET__, so if you wish, the following is valid:
 
      });
 
-## Using HTTP/2
-
-To make a request using HTTP/2 protocol only (with no HTTP/1.x fallback), use the `.http2()` method. Currently we do not have auto-detection of HTTP/2-capable servers. This feature is experimental.
-
-    const request = require('superagent');
-    const res = await request
-      .get('https://example.com/h2')
-      .http2();
-
 ## Setting header fields
 
 Setting header fields is simple, invoke `.set()` with a field name and value: