travis: don't create source and wheel distributions until 'deploy' stage

Signed-off-by: Graeme Watt <Graeme.Watt@durham.ac.uk>

.travis.yml

@@ -20,7 +20,7 @@ install:
   - "travis_retry pip install --upgrade -r requirements.txt"
 
 script:
-  - python setup.py install sdist bdist_wheel
+  - python setup.py install
   - pytest testsuite
 
 after_success:
@@ -29,7 +29,6 @@ after_success:
 sudo: false
 
 deploy:
-  skip_cleanup: true
   skip_existing: true
   provider: pypi
   user: "__token__"