Stop testing on node 7 due to unfixed bugs

Node 7 does not include nodejs/node#11015.
Without this fix, the process is killed when proxying a connection
through HTTPS. We could try to work around this on our end, but node 7
is officially unsupported, and it seems more sensible to simply drop
support here entirely too.

.travis.yml

@@ -3,7 +3,6 @@ language: node_js
 node_js:
     - 'node'
     - '8'
-    - '7'
     - '6'
 before_script:
     - "sudo chown root /opt/google/chrome/chrome-sandbox"

README.md

@@ -26,7 +26,8 @@ Ok, let's summarize. Mockttp lets you:
 
 * Write **easy, fast & reliable node.js & browser HTTP integration tests**
 * Fake server responses and verify requests made by your code
-* **Mock HTTP requests from inside & outside your process/browser tab**, including subprocesses, native code, remote devices, and more.
+* **Intercept, mock and proxy HTTPS too**, with built-in certificate generation
+* **Mock HTTP requests from inside & outside your process/tab**, including subprocesses, native code, remote devices, and more
 * Stub and mock requests transparently, as an **HTTP mocking proxy**, as well as serving traffic directly
 * **Mock servers for node & browsers with the same code** (universal/'isomorphic' HTTP mocking)
 * **Safely mock HTTP in parallel**, with autoconfiguration of ports, mock URLs and proxy settings