Add airtap browser testing

Author: lukechilds

.airtap.yml

@@ -0,0 +1,13 @@
+browsers:
+  - name: chrome
+    version: latest
+  - name: firefox
+    version: latest
+  - name: safari
+    version: latest
+  - name: ie
+    version: latest
+  - name: microsoftedge
+    version: latest
+sauce_connect: true
+loopback: airtap.local

.travis.yml

@@ -5,7 +5,15 @@ node_js:
   - '10'
   - '8'
   - '6'
-script: npm test
-notifications:
-  email:
-    on_success: never
+script:
+  - 'npm test'
+  # Run browser tests on one node version.
+  - 'if [ "${TRAVIS_PULL_REQUEST}" = "false" ] && [ "${TRAVIS_NODE_VERSION}" = "stable" ]; then npm run test:browsers; fi'
+addons:
+  sauce_connect: true
+  hosts:
+    - airtap.local
+env:
+  global:
+  - secure: qThuKBZQtkooAvzaYldECGNqvKGPRTnXx62IVyhSbFlsCY1VCmjhLldhyPDiZQ3JqL1XvSkK8OMDupiHqZnNE0nGijoO4M/kaEdjBB+jpjg3f8I6te2SNU935SbkfY9KHAaFXMZwdcq7Fk932AxWEu+FMSDM+080wNKpEATXDe4=
+  - secure: O/scKjHLRcPN5ILV5qsSkksQ7qcZQdHWEUUPItmj/4+vmCc28bHpicoUxXG5A96iHvkBbdmky/nGCg464ZaNLk68m6hfEMDAR3J6mhM2Pf5C4QI/LlFlR1fob9sQ8lztwSGOItwdK8Rfrgb30RRVV71f6FxnaJ6PKMuMNT5S1AQ=