test: configure Travis for CI

.travis.yml

@@ -0,0 +1,16 @@
+language: python
+
+python:
+    - "3.6"
+    - "3.7"
+    - "3.8"
+
+install:
+    - pip install -r requirements.txt
+    - pip install pytest pytest-cov codecov
+
+script:
+    - pytest --cov=lnbits
+
+after_success:
+    - codecov