Remove --extras option from flit install in .travis.yml

Because it installs all dependencies anyway; there is no
way to install only core and test dependencies.

.travis.yml

@@ -5,7 +5,7 @@ python:
   - 3.7
 install:
   - pip install flit
-  - flit install --symlink --extras test
+  - flit install --symlink
 script: pytest
 after_success:
   - pip install coveralls