add travis file

.travis.yml

diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..b5590b3
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,20 @@
+language: dart
+sudo: false
+dart:
+  - dev
+  - stable
+  - 1.22.1
+  - 1.21.1
+  - 1.20.1
+  - 1.19.1
+cache:
+  directories:
+    - $HOME/.pub-cache
+dart_task:
+  - test: --platform vm
+    xvfb: false
+  - test: --platform firefox
+  - test: --platform dartium
+    install_dartium: true
+  - dartfmt
+  - dartanalyzer