Fix travis configuration

Author: Cito

.travis.yml

@@ -2,7 +2,7 @@ language: python
 
 matrix:
   include:
-    - env: TOXENV=black,flake,mypy
+    - env: TOXENV=black,flake8,mypy
       python: 3.7
       dist: xenial
       sudo: true
@@ -23,7 +23,7 @@ install:
   - poetry install
 
 script:
-   - tox -e $TOX_ENV -- --cov-report term-missing --cov=graphql
+   - tox -e $TOXENV -- --cov-report term-missing --cov=graphql
 
 after_success:
    - codecov