Fix travis config

Author: Anton Prokhorov

.travis.yml

@@ -18,9 +18,9 @@ matrix:
  - python: '3.3'
  env: PACKAGES="django>=1.10,<1.11"
  - python: '3.5'
- env: PACKAGES="django>=1.7,<1.8"
+ env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8"
  - python: '3.6'
- env: PACKAGES="django>=1.7,<1.8"
+ env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8"
 before_install:
 - pip install codecov
 install: