Remove django-migration-linter which was causing strange errors

.travis.yml

@@ -48,10 +48,6 @@ script:
     - rm inventree_default_db.sqlite3
     - invoke migrate
     - invoke import-records -f data.json
-    # Run linting checks on migration files (django-migration-linter)
-    # Run subset of linting checks on *ALL* migration files
-    # Run strict migration file checks on *NEW* migrations (old ones are what they are)
-    - cd InvenTree && python manage.py lintmigrations 79ddea50f507e34195bad635008419daac0d7a5f -q ok ignore --no-cache && cd ..
 
 after_success:
     - coveralls

InvenTree/InvenTree/settings.py

@@ -211,7 +211,6 @@ INSTALLED_APPS = [
     'djmoney',                              # django-money integration
     'djmoney.contrib.exchange',             # django-money exchange rates
     'error_report',                         # Error reporting in the admin interface
-    'django_migration_linter',              # Linting checking for migration files
 ]
 
 MIDDLEWARE = CONFIG.get('middleware', [

requirements.txt

@@ -28,7 +28,6 @@ django-money==1.1               # Django app for currency management
 certifi                         # Certifi is (most likely) installed through one of the requirements above
 django-error-report==0.2.0      # Error report viewer for the admin interface
 django-test-migrations==1.1.0   # Unit testing for database migrations 
-django-migration-linter==2.5.0  # Linting checks for database migrations
 python-barcode[images]==0.13.1  # Barcode generator
 qrcode[pil]==6.1                # QR code generator