fix: use nodejs language in travis-ci (#177)

* fix: use nodejs language in travis-ci

* Update .travis.yml

* fix: travis cache

Author: njzjz

.travis.yml

@@ -1,13 +1,13 @@
-language: c
+language: node_js
 os: linux
-sudo: false
+node_js: 11
 env: PYTHON_VERSION=3.7 SETUP_XVFB=True PIP_DEPENDENCIES="tox-conda tox-travis" CONDA_CHANNEL_PRIORITY=True CONDA_CHANNELS="conda-forge"
 cache:
-- pip
-- yarn
+  yarn: true
+  directories:
+  - $HOME/.cache/pip
 services: xvfb
 install:
-- nvm i 11
 - git clone --depth 1 git://github.com/astropy/ci-helpers.git && source ci-helpers/travis/setup_conda.sh
 - yarn
 script: