Update .travis.yml and CONTRIBUTING.md

Remove the parts that apply only to the Go code and get CI working on
the new directory structure.

.travis.yml

-language: go
+language: node_js
 
-dist: xenial
-
-go_import_path: git.torproject.org/pluggable-transports/snowflake.git
-
-addons:
-    apt:
-        sources:
-            - ubuntu-toolchain-r-test
-        packages:
-            - g++-5
-            - gcc-5
+node_js:
+    - 8
 
-go:
-    - 1.13.x
-
-env:
-    - TRAVIS_NODE_VERSION="8" CC="gcc-5" CXX="g++-5"
-
-before_install:
-    - nvm install $TRAVIS_NODE_VERSION
+dist: xenial
 
 install:
-    - pushd proxy
     - npm install
-    - popd
 
 script:
-    - test -z "$(go fmt ./...)"
-    - go vet ./...
-    - go test -v -race ./...
-    - cd proxy
     - npm run lint
     - npm test

CONTRIBUTING.md

-
-- When editing Go, please run `go fmt` before every commit.
-
-- You may run tests locally with either `go test` or `npm test` for Go and
-  JavaScript, respectively.
+- You may run tests locally with `npm test`