Travis CI Configuration and README badge

This commit adds travis-ci.com configuration and the related build status badge to the README header.

.travis.yml

@@ -0,0 +1,30 @@
+language: php
+
+env:
+  global:
+    - setup=stable
+
+matrix:
+  fast_finish: true
+  include:
+    - php: 7.1
+    - php: 7.1
+      env: setup=lowest
+    - php: 7.2
+    - php: 7.2
+      env: setup=lowest
+    - php: 7.3
+    - php: 7.3
+      env: setup=lowest
+
+sudo: false
+
+cache:
+  directories:
+    - $HOME/.composer/cache
+
+install:
+  - if [[ $setup = 'stable' ]]; then travis_retry composer update --prefer-dist --no-interaction --prefer-stable --no-suggest; fi
+  - if [[ $setup = 'lowest' ]]; then travis_retry composer update --prefer-dist --no-interaction --prefer-lowest --prefer-stable --no-suggest; fi
+
+script: vendor/bin/phpunit