Remove wheel from dependencies

I might be wrong here, but i think wheel should only be needed during
upload time for bdist_wheel. Having it in requirements.txt like that
means that pbr will pick it up as an actual dependency.

.travis.yml

@@ -9,7 +9,7 @@ python:
   - "pypy"
 
 # command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
-install: pip install coverage coveralls -r requirements.txt -r test-requirements.txt
+install: pip install coverage coveralls wheel -r requirements.txt -r test-requirements.txt
 
 # command to run tests, e.g. python setup.py test
 script: coverage run -m testtools.run discover

dev-requirements.txt

@@ -0,0 +1,3 @@
+coverage
+sphinx
+wheel

requirements.txt

@@ -1,3 +1,2 @@
 requests
 six
-wheel==0.22.0